1. 8 Web application APIs
    1. 8.1 Scripting
      1. 8.1.1 Introduction
      2. 8.1.2 Enabling and disabling scripting
      3. 8.1.3 Processing model
        1. 8.1.3.1 Definitions
        2. 8.1.3.2 Fetching scripts
        3. 8.1.3.3 Creating scripts
        4. 8.1.3.4 Calling scripts
        5. 8.1.3.5 Realms, settings objects, and global objects
          1. 8.1.3.5.1 Entry
          2. 8.1.3.5.2 Incumbent
          3. 8.1.3.5.3 Current
          4. 8.1.3.5.4 Relevant
        6. 8.1.3.6 Killing scripts
        7. 8.1.3.7 Integration with the JavaScript job queue
          1. 8.1.3.7.1 EnqueueJob(queueName, job, arguments)
        8. 8.1.3.8 Integration with the JavaScript module system
          1. 8.1.3.8.1 HostResolveImportedModule(referencingModule, specifier)
        9. 8.1.3.9 Integration with the JavaScript agent formalism
        10. 8.1.3.10 Integration with the JavaScript agent cluster formalism
        11. 8.1.3.11 Runtime script errors
          1. 8.1.3.11.1 Runtime script errors in documents
          2. 8.1.3.11.2 The ErrorEvent interface
        12. 8.1.3.12 Unhandled promise rejections
          1. 8.1.3.12.1 HostPromiseRejectionTracker(promise, operation)
          2. 8.1.3.12.2 The PromiseRejectionEvent interface
        13. 8.1.3.13 HostEnsureCanCompileStrings(callerRealm, calleeRealm)
      4. 8.1.4 Event loops
        1. 8.1.4.1 Definitions
        2. 8.1.4.2 Processing model
        3. 8.1.4.3 Generic task sources
        4. 8.1.4.4 Dealing with the event loop from other specifications
      5. 8.1.5 Events
        1. 8.1.5.1 Event handlers
        2. 8.1.5.2 Event handlers on elements, Document objects, and Window objects
          1. 8.1.5.2.1 IDL definitions
        3. 8.1.5.3 Event firing
    2. 8.2 The WindowOrWorkerGlobalScope mixin
    3. 8.3 Base64 utility methods

8 Web application APIs

8.1 Scripting

8.1.1 Introduction

Various mechanisms can cause author-provided executable code to run in the context of a document. These mechanisms include, but are probably not limited to:

8.1.2 Enabling and disabling scripting

Scripting is enabled in a browsing context when all of the following conditions are true:

Scripting is disabled in a browsing context when any of the above conditions are false (i.e. when scripting is not enabled).


Scripting is enabled for a node if the node's node document has a browsing context, and scripting is enabled in that browsing context.

Scripting is disabled for a node if there is no such browsing context, or if scripting is disabled in that browsing context.

8.1.3 Processing model

8.1.3.1 Definitions

A script is one of two possible structures. All scripts have:

A settings object

An environment settings object, containing various settings that are shared with other scripts in the same context.

A classic script is a script that has the following additional fields:

Source text

A string containing a block of executable code to be evaluated as a JavaScript Script.

Optionally, a muted errors flag

A flag which, if set, means that error information will not be provided for errors in this script (used to mute errors for cross-origin scripts, since that can leak private information).

A module script is a script that has the following additional fields:

A module record

A Source Text Module Record representing the parsed module, ready to be evaluated.

A base URL

A base URL used for resolving module specifiers when resolving a module specifier. This will either be the URL from which the script was obtained, for external module scripts, or the document base URL of the containing document, for inline module scripts.

A state

One of "uninstantiated", "errored", or "instantiated", used to prevent reinvocation of ModuleDeclarationInstantiation on modules that failed to instantiate previously, and to ensure errors during parsing, instantiation, or evaluation are remembered and propagated correctly.

An error

A JavaScript value, which has meaning only if the state is "errored".

A credentials mode

A credentials mode used to fetch imported modules.

A cryptographic nonce

A cryptographic nonce used to fetch imported modules.

A parser state

The parser metadata used to fetch imported modules.

To error a module script script with a given value error, perform the following steps:

  1. Assert: script's state is not "errored".

  2. If script's module record is set, then:

    1. Set script module record's [[HostDefined]] field to undefined.

    2. Set script's module record to null.

  3. Set script's state to "errored".

  4. Set script's error to error.


An environment is an object that identifies the settings of a current or potential execution environment. An environment has the following fields:

An id

An opaque string that uniquely identifies the environment.

A creation URL

A URL record that represents the location of the resource with which the environment is associated.

In the case of an environment settings object, this URL might be distinct from the environment settings object's responsible document's URL, due to mechanisms such as history.pushState().

A target browsing context

Null or a target browsing context for a navigation request.

An active service worker

Null or a service worker that controls the environment.

An execution ready flag

A flag that indicates whether the environment setup is done. It is initially unset.

An environment settings object is an environment that additionally specifies algorithms for:

A realm execution context

A JavaScript execution context shared by all scripts that use this settings object, i.e. all scripts in a given JavaScript realm. When we run a classic script or run a module script, this execution context becomes the top of the JavaScript execution context stack, on top of which another execution context specific to the script in question is pushed. (This setup ensures ParseScript and ModuleEvaluation know which Realm to use.)

A module map

A module map that is used when importing JavaScript modules.

A responsible browsing context

A browsing context that is assigned responsibility for actions taken by the scripts that use this environment settings object.

When a script creates and navigates a new top-level browsing context, the opener attribute of the new browsing context's Window object will be set to the responsible browsing context's WindowProxy object.

A responsible event loop

An event loop that is used when it would not be immediately clear what event loop to use.

A responsible document

A Document that is assigned responsibility for actions taken by the scripts that use this environment settings object.

For example, the URL of the responsible document is used to set the URL of the Document after it has been reset using document.open().

If the responsible event loop is not a browsing context event loop, then the environment settings object has no responsible document.

An API URL character encoding

A character encoding used to encode URLs by APIs called by scripts that use this environment settings object.

An API base URL

A URL used by APIs called by scripts that use this environment settings object to parse URLs.

An origin

An origin used in security checks.

An HTTPS state

An HTTPS state value representing the security properties of the network channel used to deliver the resource with which the environment settings object is associated.

A referrer policy

The default referrer policy for fetches performed using this environment settings object as a request client. [REFERRERPOLICY]

An environment settings object also has an outstanding rejected promises weak set and an about-to-be-notified rejected promises list, used to track unhandled promise rejections. The outstanding rejected promises weak set must not create strong references to any of its members, and implementations are free to limit its size, e.g. by removing old entries from it when new ones are added.

8.1.3.2 Fetching scripts

This section introduces a number of algorithms for fetching scripts, taking various necessary inputs and resulting in classic or module scripts.

The algorithms below can be customized by optionally supplying a custom perform the fetch hook, which takes a request and an is top-level flag. The algorithm must complete with a response (which may be a network error), either synchronously (when using fetch a classic worker-imported script) or asynchronously (otherwise). The is top-level flag will be set for all classic script fetches, and for the initial fetch when fetching a module script graph or fetching a module worker script graph, but not for the fetches resulting from import statements encountered throughout the graph.

By default, not supplying the perform the fetch will cause the below algorithms to simply fetch the given request, with algorithm-specific customizations to the request and validations of the resulting response.

To layer your own customizations on top of these algorithm-specific ones, supply a perform the fetch hook that modifies the given request, fetches it, and then performs specific validations of the resulting response (completing with a network error if the validations fail).

The hook can also be used to perform more subtle customizations, such as keeping a cache of responses and avoiding performing a fetch at all.

Service Workers is an example of a specification that runs these algorithms with its own options for the hook. [SW]

To fetch a classic script given a url, a settings object, a cryptographic nonce, an integrity metadata, a parser state, a CORS setting, and a character encoding, run these steps. The algorithm will asynchronously complete with either null (on failure) or a new classic script (on success).

  1. Let request be the result of creating a potential-CORS request given url, "script", and CORS setting.

  2. Set request's client to settings object, its type to "script", its cryptographic nonce metadata to cryptographic nonce, its integrity metadata to integrity metadata and its parser metadata to parser state.

  3. If the caller specified custom steps to perform the fetch, perform them on request, with the is top-level flag set. Return from this algorithm, and when the custom perform the fetch steps complete with response response, run the remaining steps.

    Otherwise, fetch request. Return from this algorithm, and run the remaining steps as part of the fetch's process response for the response response.

    response can be either CORS-same-origin or CORS-cross-origin. This only affects how error reporting happens.

  4. Let response be response's unsafe response.

  5. If response's type is "error", or response's status is not an ok status, asynchronously complete this algorithm with null, and abort these steps.

  6. If response's Content Type metadata, if any, specifies a character encoding, and the user agent supports that encoding, then set character encoding to that encoding (ignoring the passed-in value).

  7. Let source text be the result of decoding response's body to Unicode, using character encoding as the fallback encoding.

    The decode algorithm overrides character encoding if the file contains a BOM.

  8. Let script be the result of creating a classic script using source text and settings object.

    If response was CORS-cross-origin, then pass the muted errors flag to the create a classic script algorithm as well.

  9. Asynchronously complete this algorithm with script.

To fetch a classic worker script given a url, a fetch client settings object, a destination, and a script settings object, run these steps. The algorithm will asynchronously complete with either null (on failure) or a new classic script (on success).

  1. Let request be a new request whose url is url, client is fetch client settings object, type is "script", destination is destination, mode is "same-origin", credentials mode is "same-origin", parser metadata is "not parser-inserted", and whose use-URL-credentials flag is set.

  2. If the caller specified custom steps to perform the fetch, perform them on request, with the is top-level flag set. Return from this algorithm, and when the custom perform the fetch steps complete with response response, run the remaining steps.

    Otherwise, fetch request. Return from this algorithm, and run the remaining steps as part of the fetch's process response for the response response.

  3. Let response be response's unsafe response.

  4. If response's type is "error", or response's status is not an ok status, asynchronously complete this algorithm with null, and abort these steps.

  5. Let source text be the result of UTF-8 decoding response's body.

  6. Let script be the result of creating a classic script using source text and script settings object.

  7. Asynchronously complete this algorithm with script.

To fetch a classic worker-imported script given a url and a settings object, run these steps. The algorithm will synchronously complete with a classic script on success, or throw an exception on failure.

  1. Let request be a new request whose url is url, client is settings object, type is "script", destination is "script", parser metadata is "not parser-inserted", synchronous flag is set, and whose use-URL-credentials flag is set.

  2. If the caller specified custom steps to perform the fetch, perform them on request, with the is top-level flag set. Let response be the result.

    Otherwise, fetch request, and let response be the result.

    Unlike other algorithms in this section, the fetching process is synchronous here. Thus any perform the fetch steps will also finish their work synchronously.

  3. Let response be response's unsafe response.

  4. If response's type is "error", or response's status is not an ok status, throw a "NetworkError" DOMException and abort these steps.

  5. Let source text be the result of UTF-8 decoding response's body.

  6. Let script be the result of creating a classic script using source text and settings object.

    If response was CORS-cross-origin, then pass the muted errors flag to the create a classic script algorithm as well.

  7. Return script.

To fetch a module script graph given a url, a settings object, a destination, a cryptographic nonce, a parser state, and a credentials mode, run these steps. The algorithm will asynchronously complete with either null (on failure) or a module script (on success).

  1. Perform the internal module script graph fetching procedure given url, settings object, destination, cryptographic nonce, parser state, credentials mode, settings object, a new empty list, "client", and with the top-level module fetch flag set. If the caller of this algorithm specified custom perform the fetch steps, pass those along as well.

  2. When the internal module script graph fetching procedure asynchronously completes with result, asynchronously complete this algorithm with result.

To fetch a module worker script graph given a url, a fetch client settings object, a destination, a credentials mode, and a module map settings object, run these steps. The algorithm will asynchronously complete with either null (on failure) or a module script (on success).

  1. Perform the internal module script graph fetching procedure given url, fetch client settings object, destination, the empty string, "not parser-inserted", credentials mode, module map settings object, a new empty list, "client", and with the top-level module fetch flag set. If the caller of this algorithm specified custom perform the fetch steps, pass those along as well.

  2. When the internal module script graph fetching procedure asynchronously completes with result, asynchronously complete this algorithm with result.


The following algorithms are meant for internal use by this specification only as part of fetching a module script graph or preparing a script, and should not be used directly by other specifications.

To perform the internal module script graph fetching procedure given a url, a fetch client settings object, a destination, a cryptographic nonce, a parser state, a credentials mode, a module map settings object, an ancestor list, a referrer, and a top-level module fetch flag, perform these steps. The algorithm will asynchronously complete with either null (on failure) or a module script (on success).

  1. Fetch a single module script given url, fetch client settings object, destination, cryptographic nonce, parser state, credentials mode, module map settings object, referrer, and the top-level module fetch flag. If the caller of this algorithm specified custom perform the fetch steps, pass those along while fetching a single module script.

  2. Return from this algorithm, and run the following steps when fetching a single module script asynchronously completes with result:

  3. If result is null, asynchronously complete this algorithm with null and abort these steps.

  4. If result's state is "instantiated" or "errored", asynchronously complete this algorithm with result, and abort these steps.

  5. Assert: result's state is "uninstantiated".

  6. Fetch the descendants of and instantiate result given destination and an ancestor list obtained by appending url to ancestor list.

  7. When the fetch the descendants of and instantiate a module script algorithm asynchronously completes with final result, asynchronously complete this algorithm with final result.

To fetch a single module script, given a url, a fetch client settings object, a destination, a cryptographic nonce, a parser state, a credentials mode, a module map settings object, a referrer, and a top-level module fetch flag, run these steps. The algorithm will asynchronously complete with either null (on failure) or a module script (on success).

  1. Let moduleMap be module map settings object's module map.

  2. If moduleMap[url] is "fetching", wait in parallel until that entry's value changes, then queue a task on the networking task source to proceed with running the following steps.

  3. If moduleMap[url] exists, asynchronously complete this algorithm with moduleMap[url], and abort these steps.

  4. Set moduleMap[url] to "fetching".

  5. Let request be a new request whose url is url, destination is destination, type is "script", mode is "cors", credentials mode is credentials mode, cryptographic nonce metadata is cryptographic nonce, parser metadata is parser state, referrer is referrer, and client is fetch client settings object.

  6. If the caller specified custom steps to perform the fetch, perform them on request, setting the is top-level flag if the top-level module fetch flag is set. Return from this algorithm, and when the custom perform the fetch steps complete with response response, run the remaining steps.

    Otherwise, fetch request. Return from this algorithm, and run the remaining steps as part of the fetch's process response for the response response.

    response is always CORS-same-origin.

  7. If any of the following conditions are met, set moduleMap[url] to null, asynchronously complete this algorithm with null, and abort these steps:

  8. Let source text be the result of UTF-8 decoding response's body.

  9. Let module script be the result of creating a module script given source text, module map settings object, response's url, cryptographic nonce, parser state, and credentials mode.

  10. Set moduleMap[url] to module script, and asynchronously complete this algorithm with module script.

    It is intentional that the module map is keyed by the request URL, whereas the base URL for the module script is set to the response URL. The former is used to deduplicate fetches, while the latter is used for URL resolution.

To fetch the descendants of and instantiate a module script module script, given a destination and an optional ancestor list, run these steps. The algorithm will asynchronously complete with either null (on failure) or with module script (on success).

  1. If ancestor list was not given, let it be the empty list.

  2. If module script's state is "instantiated" or "errored", asynchronously complete this algorithm with module script, and abort these steps.

  3. Assert: module script's state is "uninstantiated".

  4. Let record be module script's module record.

  5. If record.[[RequestedModules]] is empty, asynchronously complete this algorithm with module script.

  6. Let urls be a new empty list.

  7. For each string requested of record.[[RequestedModules]],

    1. Let url be the result of resolving a module specifier given module script and requested.

    2. If url is failure:

      1. Let error be a new TypeError exception.

      2. Error module script with error.

      3. Abort this algorithm, and asynchronously complete it with module script.

    3. Otherwise, if ancestor list does not contain url, append url to urls.

  8. Let descendants result be null.

  9. For each url in urls, perform the internal module script graph fetching procedure given url, module script's credentials mode, module script's cryptographic nonce, module script's parser state, destination, module script's settings object, module script's settings object, ancestor list, module script's base URL, and with the top-level module fetch flag unset. If the caller of this algorithm specified custom perform the fetch steps, pass those along while performing the internal module script graph fetching procedure.

    These invocations of the internal module script graph fetching procedure should be performed in parallel to each other.

    If any invocation of the internal module script graph fetching procedure asynchronously completes with null, then optionally abort all other invocations, set descendants result to null, and proceed to the next step. (The un-fetched descendant will cause errors during instantiation.)

    If any invocation of the internal module script graph fetching procedure asynchronously completes with a module script whose state is "errored", then optionally abort all other invocations, set descendants result to module script, and proceed to the next step. (The errored descendant will cause errors during instantiation.)

    Otherwise, wait for all of the internal module script graph fetching procedure invocations to asynchronously complete, with module scripts whose states are not "errored". Then, set descendants result to module script, and proceed to the next step.

  10. Let instantiationStatus be record.ModuleDeclarationInstantiation().

    This step will recursively call ModuleDeclarationInstantiation all of the module's uninstantiated dependencies.

  11. For each script in module script's uninstantiated inclusive descendant module scripts, perform the following steps:

    1. If instantiationStatus is an abrupt completion, then error script with instantiationStatus.[[Value]].

    2. Otherwise, set script's state to "instantiated".

  12. Asynchronously complete this algorithm with descendants result.

    It is intentional that we complete with descendants result here, and not module script, as this allows us to notify the caller of fetching errors.

In the above algorithm, a module script script's uninstantiated inclusive descendant module scripts is a set of module scripts determined as follows:

  1. If script's module record is null, return the empty set.

  2. Let moduleMap be script's settings object's module map.

  3. Let stack be the stack « script ».

  4. Let inclusive descendants be an empty set.

  5. While stack is not empty:

    1. Let current the result of popping from stack.

    2. Assert: current is a module script (i.e., it is not "fetching" or null).

    3. If inclusive descendants and stack both do not contain current, then:

      1. Append current to inclusive descendants.

      2. Let child specifiers be the value of current's module record's [[RequestedModules]] internal slot.

      3. Let child URLs be the list obtained by calling resolve a module specifier once for each item of child specifiers, given current and that item. Omit any failures.

      4. Let child modules be the list obtained by getting each value in moduleMap whose key is given by an item of child URLs.

      5. For each s of child modules:

        1. If inclusive descendants already contains s, continue.

        2. If s is null, continue.

        3. Assert: s is a module script (i.e., it is not "fetching", since by this point all child modules must have been fetched).

        4. Push s onto stack.

  6. Return a set containing all items of inclusive descendants whose state is "uninstantiated".

The above algorithm gives a depth-first search of the module dependency graph. The main interesting part is in how the "edges" of this graph are determined. The actual search implementation is not important; any other technique for exploring the graph will suffice, given that the output is a set only used for membership testing and whose order is thus not important.

8.1.3.3 Creating scripts

To create a classic script, given some script source, an environment settings object, and an optional muted errors flag:

  1. Let script be a new classic script that this algorithm will subsequently initialize.

  2. Set script's settings object to the environment settings object provided.

  3. If scripting is disabled for the given environment settings object's responsible browsing context, then set script's source text to the empty string. Otherwise, set script's source text to the supplied script source.

  4. If the muted errors flag was set, then set script's muted errors flag.

  5. Return script.

To create a module script, given some script source, an environment settings object, a script base URL, a cryptographic nonce, a parser state, and a credentials mode:

  1. Let script be a new module script that this algorithm will subsequently initialize.

  2. Set script's settings object to the environment settings object provided.

  3. Let realm be the provided environment settings object's Realm.

  4. If scripting is disabled for the given environment settings object's responsible browsing context, then let script source be the empty string. Otherwise, let script source be the provided script source.

  5. Let result be ParseModule(script source, realm, script).

    Passing script as the last parameter here ensures result.[[HostDefined]] will be script.

  6. If result is a List of errors, then:

    1. Error script with errors[0].

    2. Return script.

  7. Set script's state to "uninstantiated".

  8. Set script's module record to result.

  9. Set script's base URL to the script base URL provided.

  10. Set script's cryptographic nonce to the cryptographic nonce provided.

  11. Set script's parser state to the parser state.

  12. Set script's credentials mode to the credentials mode provided.

  13. Return script.

8.1.3.4 Calling scripts

To run a classic script given a classic script s and an optional rethrow errors flag:

  1. Let settings be the settings object of s.

  2. Check if we can run script with settings. If this returns "do not run", then return undefined and abort these steps.

  3. Let realm be settings's Realm.

  4. Prepare to run script with settings.

  5. Let result be ParseScript(s's source text, realm, s).

    Passing s as the last parameter here ensures result.[[HostDefined]] will be s.

  6. If result is a List of errors, set result to the first element of result and go to the step labeled error.

  7. Let evaluationStatus be ScriptEvaluation(result).

  8. If evaluationStatus is an abrupt completion, set result to evaluationStatus.[[Value]] and go to the next step (labeled error). If evaluationStatus is a normal completion, or if ScriptEvaluation does not complete because the user agent has aborted the running script, skip to the step labeled cleanup.

  9. Error: At this point result must be an exception. Perform the following steps:

    1. If the rethrow errors flag is set and s's muted errors flag is not set, rethrow result.

    2. If the rethrow errors flag is set and s's muted errors flag is set, throw a "NetworkError" DOMException.

    3. If the rethrow errors flag is not set, report the exception given by result for the script s.

  10. Cleanup: Clean up after running script with settings.

  11. If evaluationStatus exists and is a normal completion, return evaluationStatus.[[Value]]. Otherwise, script execution was unsuccessful, either because an error occurred during parsing, or an exception occurred during evaluation, or because it was aborted prematurely.

To run a module script given a module script s:

  1. Let settings be the settings object of s.

  2. Check if we can run script with settings. If this returns "do not run" then abort these steps.

  3. If s's state is "errored", then report the exception given by s's error for s and abort these steps.

  4. Assert: s's state is "instantiated" (and thus its module record is not null).

  5. Let record be s's module record.

  6. Prepare to run script given settings.

  7. Let evaluationStatus be record.ModuleEvaluation().

    This step will recursively evaluate all of the module's dependencies.

    If ModuleEvaluation fails to complete as a result of the user agent aborting the running script, then set evaluationStatus to Completion { [[Type]]: throw, [[Value]]: a new "QuotaExceededError" DOMException, [[Target]]: empty }.

  8. If evaluationStatus is an abrupt completion, then:

    1. Error script with evaluationStatus.[[Value]].

    2. Report the exception given by evaluationStatus.[[Value]] for s.

  9. Clean up after running script with settings.

The steps to check if we can run script with an environment settings object settings are as follows. They return either "run" or "do not run".

  1. If the global object specified by settings is a Window object whose Document object is not fully active, then return "do not run" and abort these steps.

  2. If scripting is disabled for the responsible browsing context specified by settings, then return "do not run" and abort these steps.

  3. Return "run".

The steps to prepare to run script with an environment settings object settings are as follows:

  1. Push settings's realm execution context onto the JavaScript execution context stack; it is now the running JavaScript execution context.

The steps to clean up after running script with an environment settings object settings are as follows:

  1. Assert: settings's realm execution context is the running JavaScript execution context.

  2. Remove settings's realm execution context from the JavaScript execution context stack.

  3. If the JavaScript execution context stack is now empty, perform a microtask checkpoint. (If this runs scripts, these algorithms will be invoked reentrantly.)

These algorithms are not invoked by one script directly calling another, but they can be invoked reentrantly in an indirect manner, e.g. if a script dispatches an event which has event listeners registered.

The running script is the script in the [[HostDefined]] field in the ScriptOrModule component of the running JavaScript execution context.

8.1.3.5 Realms, settings objects, and global objects

A global object is a JavaScript object that is the [[GlobalObject]] field of a JavaScript realm.

In this specification, all JavaScript realms are initialized with global objects that are either Window or WorkerGlobalScope objects.

There is always a 1-to-1-to-1 mapping between JavaScript realms, global objects, and environment settings objects:


When defining algorithm steps throughout this specification, it is often important to indicate what JavaScript realm is to be used—or, equivalently, what global object or environment settings object is to be used. In general, there are at least four possibilities:

Entry
This corresponds to the script that initiated the currently running script action: i.e., the function or script that the user agent called into when it called into author code.
Incumbent
This corresponds to the most-recently-entered author function or script on the stack, or the author function or script that originally scheduled the currently-running callback.
Current
This corresponds to the currently-running function object, including built-in user-agent functions which might not be implemented as JavaScript. (It is derived from the current JavaScript realm.)
Relevant
Every platform object has a relevant Realm, which is roughly the JavaScript realm in which it was created. When writing algorithms, the most prominent platform object whose relevant Realm might be important is the this value of the currently-running function object. In some cases, there can be other important relevant Realms, such as those of any arguments.

Note how the entry, incumbent, and current concepts are usable without qualification, whereas the relevant concept must be applied to a particular platform object.

Consider the following pages, with a.html being loaded in a browser window, b.html being loaded in an iframe as shown, and c.html and d.html omitted (they can simply be empty documents):

<!-- a.html -->
<!DOCTYPE html>
<html lang="en">
<title>Entry page</title>

<iframe src="b.html"></iframe>
<button onclick="frames[0].hello()">Hello</button>

<!--b.html -->
<!DOCTYPE html>
<html lang="en">
<title>Incumbent page</title>

<iframe src="c.html" id="c"></iframe>
<iframe src="d.html" id="d"></iframe>

<script>
  const c = document.querySelector("#c").contentWindow;
  const d = document.querySelector("#d").contentWindow;

  window.hello = () => {
    c.print.call(d);
  };
</script>

Each page has its own browsing context, and thus its own JavaScript realm, global object, and environment settings object.

When the print() method is called in response to pressing the button in a.html, then:

The incumbent and entry concepts should not be used by new specifications, as they are excessively complicated and unintuitive to work with. We are working to remove almost all existing uses from the platform: see issue #1430 for incumbent, and issue #1431 for entry.

In general, web platform specifications should use the relevant concept, applied to the object being operated on (usually the this value of the current method). This mismatches the JavaScript specification, where current is generally used as the default (e.g. in determining the JavaScript realm whose Array constructor should be used to construct the result in Array.prototype.map). But this inconsistency is so embedded in the platform that we have to accept it going forward.

Note that in constructors, where there is no this value yet, the current concept is the appropriate default.

One reason why the relevant concept is generally a better default choice than the current concept is that it is more suitable for creating an object that is to be persisted and returned multiple times. For example, the navigator.getBattery() method creates promises in the relevant Realm for the Navigator object on which it is invoked. This has the following impact: [BATTERY]

<!-- outer.html -->
<!DOCTYPE html>
<html lang="en">
<title>Relevant Realm demo: outer page</title>
<script>
  function doTest() {
    const promise = navigator.getBattery.call(frames[0].navigator);

    console.log(promise instanceof Promise);           // logs false
    console.log(promise instanceof frames[0].Promise); // logs true

    frames[0].hello();
  }
</script>
<iframe src="inner.html" onload="doTest()"></iframe>

<!-- inner.html -->
<!DOCTYPE html>
<html lang="en">
<title>Relevant Realm demo: inner page</title>
<script>
  function hello() {
    const promise = navigator.getBattery();

    console.log(promise instanceof Promise);        // logs true
    console.log(promise instanceof parent.Promise); // logs false
  }
</script>

If the algorithm for the getBattery() method had instead used the current Realm, all the results would be reversed. That is, after the first call to getBattery() in outer.html, the Navigator object in inner.html would be permanently storing a Promise object created in outer.html's JavaScript realm, and calls like that inside the hello() function would thus return a promise from the "wrong" realm. Since this is undesirable, the algorithm instead uses the relevant Realm, giving the sensible results indicated in the comments above.


The rest of this section deals with formally defining the entry, incumbent, current, and relevant concepts.

8.1.3.5.1 Entry

The process of calling scripts will push or pop realm execution contexts onto the JavaScript execution context stack, interspersed with other execution contexts.

With this in hand, we define the entry execution context to be the most recently pushed item in the JavaScript execution context stack that is a realm execution context. The entry Realm is the entry execution context's Realm component.

Then, the entry settings object is the environment settings object of the entry Realm.

Similarly, the entry global object is the global object of the entry Realm.

8.1.3.5.2 Incumbent

All JavaScript execution contexts must contain, as part of their code evaluation state, a skip-when-determining-incumbent counter value, which is initially zero. In the process of preparing to run a callback and cleaning up after running a callback, this value will be incremented and decremented.

Every event loop has an associated backup incumbent settings object stack, initially empty. Roughly speaking, it is used to determine the incumbent settings object when no author code is on the stack, but author code is responsible for the current algorithm having been run in some way. The process of preparing to run a callback and cleaning up after running a callback manipulate this stack. [WEBIDL]

When Web IDL is used to invoke author code, or when EnqueueJob invokes a promise job, they use the following algorithms to track relevant data for determining the incumbent settings object:

To prepare to run a callback with an environment settings object settings:

  1. Push settings onto the backup incumbent settings object stack.

  2. Let context be the topmost script-having execution context.

  3. If context is not null, increment context's skip-when-determining-incumbent counter.

To clean up after running a callback with an environment settings object settings:

  1. Let context be the topmost script-having execution context.

    This will be the same as the topmost script-having execution context inside the corresponding invocation of prepare to run a callback.

  2. If context is not null, decrement context's skip-when-determining-incumbent counter.

  3. Assert: the topmost entry of the backup incumbent settings object stack is settings.

  4. Remove settings from the backup incumbent settings object stack.

Here, the topmost script-having execution context is the topmost entry of the JavaScript execution context stack that has a non-null ScriptOrModule component, or null if there is no such entry in the JavaScript execution context stack.

With all this in place, the incumbent settings object is determined as follows:

  1. Let context be the topmost script-having execution context.

  2. If context is null, or if context's skip-when-determining-incumbent counter is greater than zero, then:

    1. Assert: the backup incumbent settings object stack is not empty.

      This assert would fail if you try to obtain the incumbent settings object from inside an algorithm that was triggered neither by calling scripts nor by Web IDL invoking a callback. For example, it would trigger if you tried to obtain the incumbent settings object inside an algorithm that ran periodically as part of the event loop, with no involvement of author code. In such cases the incumbent concept cannot be used.

    2. Return the topmost entry of the backup incumbent settings object stack.

  3. Return context's Realm component's settings obect.

Then, the incumbent Realm is the Realm of the incumbent settings object.

Similarly, the incumbent global object is the global object of the incumbent settings object.


The following series of examples is intended to make it clear how all of the different mechanisms contribute to the definition of the incumbent concept:

Consider the following very simple example:

<!DOCTYPE html>
<iframe></iframe>
<script>
  new frames[0].MessageChannel();
</script>

When the MessageChannel() constructor looks up the incumbent settings object to use as the owner of the new MessagePort objects, the topmost script-having execution context will be that corresponding to the script element: it was pushed onto the JavaScript execution context stack as part of ScriptEvaluation during the run a classic script algorithm. Since there are no Web IDL callback invocations involved, the context's skip-when-determining-incumbent counter is zero, so it is used to determine the incumbent settings object; the result is the environment settings object of window.

(In this example, the environment settings object of frames[0] is not involved at all. It is the current settings object, but the MessageChannel() constructor cares only about the incumbent, not current.)

Consider the following more complicated example:

<!DOCTYPE html>
<iframe></iframe>
<script>
  const bound = frames[0].postMessage.bind(frames[0], "some data", "*");
  window.setTimeout(bound);
</script>

There are two interesting environment settings objects here: that of window, and that of frames[0]. Our concern is: what is the incumbent settings object at the time that the algorithm for postMessage() executes?

It should be that of window, to capture the intuitive notion that the author script responsible for causing the algorithm to happen is executing in window, not frames[0]. Another way of capturing the intuition here is that invoking algorithms asynchronously (in this case via setTimeout()) should not change the incumbent concept.

Let us now explain how the steps given above give us our intuitively-desired result of window's relevant settings object.

When bound is converted to a Web IDL callback type, the incumbent settings object is that corresponding to window (in the same manner as in our simple example above). Web IDL stores this as the resulting callback value's callback context.

When the task posted by setTimeout() executes, the algorithm for that task uses Web IDL to invoke the stored callback value. Web IDL in turn calls the above prepare to run a callback algorithm. This pushes the stored callback context onto the backup incumbent settings object stack. At this time (inside the timer task) there is no author code on the stack, so the topmost script-having execution context is null, and nothing gets its skip-when-determining-incumbent counter incremented.

Invoking the callback then calls bound, which in turn calls the postMessage() method of frames[0]. When the postMessage() algorithm looks up the incumbent settings object, there is still no author code on the stack, since the bound function just directly calls the built-in method. So the topmost script-having execution context will be null: the JavaScript execution context stack only contains an execution context corresponding to postMessage(), with no ScriptEvaluation context or similar below it.

This is where we fall back to the backup incumbent settings object stack. As noted above, it will contain as its topmost entry the relevant settings object of window. So that is what is used as the incumbent settings object while executing the postMessage() algorithm.

Consider this final, even more convoluted example:

<!-- a.html -->
<!DOCTYPE html>
<button>click me</button>
<iframe></iframe>
<script>
const bound = frames[0].location.assign.bind(frames[0].location, "https://example.com/");
document.querySelector("button").addEventListener("click", bound);
</script>

<!-- b.html -->
<!DOCTYPE html>
<iframe src="a.html"></iframe>
<script>
  const iframe = document.querySelector("iframe");
  iframe.onload = function onLoad() {
    iframe.contentWindow.document.querySelector("button").click();
  };
</script>

Again there are two interesting environment settings objects in play: that of a.html, and that of b.html. When the location.assign() method triggers the Location-object navigate algorithm, what will be the incumbent settings object? As before, it should intuitively be that of a.html: the click listener was originally scheduled by a.html, so even if something involving b.html causes the listener to fire, the incumbent responsible is that of a.html.

The callback setup is similar to the previous example: when bound is converted to a Web IDL callback type, the incumbent settings object is that corresponding to a.html, which is stored as the callback's callback context.

When the click() method is called inside b.html, it dispatches a click event on the button that is inside a.html. This time, when the prepare to run a callback algorithm executes as part of event dispatch, there is author code on the stack; the topmost script-having execution context is that of the onLoad function, whose skip-when-determining-incumbent counter gets incremented. Additionally, a.html's environment settings object (stored as the EventHandler's callback context) is pushed onto the backup incumbent settings object stack.

Now, when the Location-object navigate algorithm looks up the incumbent settings object, the topmost script-having execution context is still that of the onLoad function (due to the fact we are using a bound function as the callback). Its skip-when-determining-incumbent counter value is one, however, so we fall back to the backup incumbent settings object stack. This gives us the environment settings object of a.html, as expected.

Note that this means that even though it is the iframe inside a.html that navigates, it is a.html itself that is used as the source browsing context, which determines among other things the request client. This is perhaps the only justifiable use of the incumbent concept on the web platform; in all other cases the consequences of using it are simply confusing and we hope to one day switch them to use current or relevant as appropriate.

8.1.3.5.3 Current

The JavaScript specification defines the current Realm Record, sometimes abbreviated to the "current Realm". [JAVASCRIPT]

Then, the current settings object is the environment settings object of the current Realm Record.

Similarly, the current global object is the global object of the current Realm Record.

8.1.3.5.4 Relevant

The relevant settings object for a platform object is defined as follows:

If the object is a global object
Each global object in this specification is created alongside a corresponding environment settings object; that is its relevant settings object.
Otherwise

The relevant settings object for a non-global platform object o is the environment settings object whose global object is the global object of the global environment associated with o.

The "global environment associated with" concept is from the olden days, before the modern JavaScript specification and its concept of realms. We expect that as the Web IDL specification gets updated, every platform object will have a Realm associated with it, and this definition can be re-cast in those terms. [JAVASCRIPT] [WEBIDL]

Then, the relevant Realm for a platform object is the Realm of its relevant settings object.

Similarly, the relevant global object for a platform object is the global object of its relevant settings object.

8.1.3.6 Killing scripts

Although the JavaScript specification does not account for this possibility, it's sometimes necessary to abort a running script. This causes any ScriptEvaluation or ModuleEvaluation to cease immediately, emptying the JavaScript execution context stack without triggering any of the normal mechanisms like finally blocks. [JAVASCRIPT]

User agents may impose resource limitations on scripts, for example CPU quotas, memory limits, total execution time limits, or bandwidth limitations. When a script exceeds a limit, the user agent may either throw a "QuotaExceededError" DOMException, abort the script without an exception, prompt the user, or throttle script execution.

For example, the following script never terminates. A user agent could, after waiting for a few seconds, prompt the user to either terminate the script or let it continue.

<script>
 while (true) { /* loop */ }
</script>

User agents are encouraged to allow users to disable scripting whenever the user is prompted either by a script (e.g. using the window.alert() API) or because of a script's actions (e.g. because it has exceeded a time limit).

If scripting is disabled while a script is executing, the script should be terminated immediately.

User agents may allow users to specifically disable scripts just for the purposes of closing a browsing context.

For example, the prompt mentioned in the example above could also offer the user with a mechanism to just close the page entirely, without running any unload event handlers.

8.1.3.7 Integration with the JavaScript job queue

The JavaScript specification defines the JavaScript job and job queue abstractions in order to specify certain invariants about how promise operations execute with a clean JavaScript execution context stack and in a certain order. However, as of the time of this writing the definition of EnqueueJob in that specification is not sufficiently flexible to integrate with HTML as a host environment. [JAVASCRIPT]

This is not strictly true. It is in fact possible, by taking liberal advantage of the many "implementation defined" sections of the algorithm, to contort it to our purposes. However, the end result is a mass of messy indirection and workarounds that essentially bypasses the job queue infrastructure entirely, albeit in a way that is technically sanctioned within the bounds of implementation-defined behavior. We do not take this path, and instead introduce the following willful violation.

As such, user agents must instead use the following definition in place of that in the JavaScript specification. These ensure that the promise jobs enqueued by the JavaScript specification are properly integrated into the user agent's event loops.

The RunJobs abstract operation from the JavaScript specification must not be used by user agents.

8.1.3.7.1 EnqueueJob(queueName, job, arguments)

When the JavaScript specification says to call the EnqueueJob abstract operation, the following algorithm must be used in place of JavaScript's EnqueueJob:

  1. Assert: queueName is "PromiseJobs". ("ScriptJobs" must not be used by user agents.)

  2. Let job settings be some appropriate environment settings object.

    It is not yet clear how to specify the environment settings object that should be used here. In practice, this means that the entry concept is not correctly specified while executing a job. See discussion in issue #1189.

  3. Let incumbent settings be the incumbent settings object.

  4. Queue a microtask, on job settings's responsible event loop, to perform the following steps:

    1. Check if we can run script with job settings. If this returns "do not run" then abort these steps.

    2. Prepare to run script with job settings.

    3. Prepare to run a callback with incumbent settings.

    4. Let result be the result of performing the abstract operation specified by job, using the elements of arguments as its arguments.

    5. Clean up after running a callback with incumbent settings.

    6. Clean up after running script with job settings.

    7. If result is an abrupt completion, report the exception given by result.[[Value]].

8.1.3.8 Integration with the JavaScript module system

The JavaScript specification defines a syntax for modules, as well as some host-agnostic parts of their processing model. This specification defines the rest of their processing model: how the module system is bootstrapped, via the script element with type attribute set to "module", and how modules are fetched, resolved, and executed. [JAVASCRIPT]

Although the JavaScript specification speaks in terms of "scripts" versus "modules", in general this specification speaks in terms of classic scripts versus module scripts, since both of them use the script element.

A module map is a map of URL records to values that are either a module script, null (used to represent failed fetches), or a placeholder value "fetching". Module maps are used to ensure that imported JavaScript modules are only fetched, parsed, and evaluated once per Document or worker.

Since module maps are keyed by URL, the following code will create three separate entries in the module map, since it results in three different URLs:

import "https://example.com/module.js";
import "https://example.com/module.js#map-buster";
import "https://example.com/module.js?debug=true";

That is, URL queries and fragments can be varied to create distinct entries in the module map; they are not ignored. Thus, three separate fetches and three separate module evaluations will be performed.

In contrast, the following code would only create a single entry in the module map, since after applying the URL parser to these inputs, the resulting URL records are equal:

import "https://example.com/module2.js";
import "https:example.com/module2.js";
import "https://///example.com\\module2.js";
import "https://example.com/foo/../module2.js";

So in this second example, only one fetch and one module evaluation will occur.

Note that this behavior is the same as how shared workers are keyed by their parsed constructor url.

To resolve a module specifier given a module script script and a string specifier, perform the following steps. It will return either a URL record or failure.

  1. Apply the URL parser to specifier. If the result is not failure, return the result.

  2. If specifier does not start with the character U+002F SOLIDUS (/), the two-character sequence U+002E FULL STOP, U+002F SOLIDUS (./), or the three-character sequence U+002E FULL STOP, U+002E FULL STOP, U+002F SOLIDUS (../), return failure and abort these steps.

    This restriction is in place so that in the future we can allow custom module loaders to give special meaning to "bare" import specifiers, like import "jquery" or import "web/crypto". For now any such imports will fail, instead of being treated as relative URLs.

  3. Return the result of applying the URL parser to specifier with script's base URL as the base URL.

The following are valid module specifiers according to the above algorithm:

The following are valid module specifiers according to the above algorithm, but will invariably cause failures when they are fetched:

The following are not valid module specifiers according to the above algorithm:

8.1.3.8.1 HostResolveImportedModule(referencingModule, specifier)

JavaScript contains an implementation-defined HostResolveImportedModule abstract operation. User agents must use the following implementation: [JAVASCRIPT]

  1. Let referencing module script be referencingModule.[[HostDefined]].

  2. Let moduleMap be referencing module script's settings object's module map.

  3. Let url be the result of resolving a module specifier given referencing module script and specifier.

  4. Assert: url is never failure, because resolving a module specifier must have been previously successful with these same two arguments during the appropriate invocation of fetch the descendants of and instantiate a module script.

  5. Let resolved module script be moduleMap[url]. (This entry must exist for us to have gotten to this point.)

  6. If resolved module script is null, then throw a TypeError exception and abort these steps.

    This occurs when we have previously tried to fetch url, and failed, but are now rediscovering that fact in a new module script graph. For example, given a file module.js whose contents are

    import "./404.js";

    then we could get here as part of fetching the graph for the second script element in the following HTML:

    <script type="module" src="404.js"></script>
    <script type="module" src="module.js"></script>
  7. Assert: resolved module script is a module script (i.e., is not "fetching").

  8. If resolved module script's state is "errored", then throw resolved module script's error.

  9. Assert: resolved module script's module record is not null.

  10. Return resolved module script's module record.

8.1.3.9 Integration with the JavaScript agent formalism

JavaScript defines the concept of an agent. Until such a time that this standard has a better handle on lifetimes, we define five types of agents that user agents must allocate at the appropriate time.

In the future, when this specification has a better handle on lifetimes, we hope to define exactly when agents and agent clusters are created.

JavaScript is expected to define agents in more detail; in particular that they hold a set of realms: tc39/ecma262 issue #882.

Similar-origin window agent

An agent whose [[CanBlock]] is false and whose set of realms consists of all realms of Window objects whose relevant settings object's responsible browsing context is in the same unit of related similar-origin browsing contexts.

Two Window objects that are same origin can be in different similar-origin window agents, for instance if they are each in their own unit of related similar-origin browsing contexts.

Dedicated worker agent

An agent whose [[CanBlock]] is true and whose set of realms consists of a single DedicatedWorkerGlobalScope object's Realm.

Shared worker agent

An agent whose [[CanBlock]] is true and whose set of realms consists a single SharedWorkerGlobalScope object's Realm.

Service worker agent

An agent whose [[CanBlock]] is false and whose set of realms consists of a single ServiceWorkerGlobalScope object's Realm.

Worklet agent

An agent whose [[CanBlock]] is false and whose set of realms consists of a single WorkletGlobalScope object's Realm.

While conceptually it might be cleaner for worklets that end up with multiple realms to put all those in the same agent, it is not observable in practice.

8.1.3.10 Integration with the JavaScript agent cluster formalism

Can share memory with defines an equivalence relation. An agent cluster consists of all agents in the same equivalence class with respect to the can share memory with equivalence relation.

A similar-origin window agent, dedicated worker agent, shared worker agent, or service worker agent, agent, can share memory with any dedicated worker agent whose single realm's global object's owner set contains an item whose relevant Realm belongs to agent.

We use item above as an owner set can contain Document objects.

A worklet agent … currently worklets have no clearly defined owner, see: w3c/css-houdini-drafts issue #224.

In addition, any agent A can share memory with:

The agent cluster concept is crucial for defining the JavaScript memory model, and in particular among which agents the backing data of SharedArrayBuffer objects can be shared.

The following pairs of global objects are each within the same agent cluster, and thus can use SharedArrayBuffer instances to share memory with each other:

The following pairs of global objects are not within the same agent cluster, and thus cannot share memory:

8.1.3.11 Runtime script errors

When the user agent is required to report an error for a particular script script with a particular position line:col, using a particular target target, it must run these steps, after which the error is either handled or not handled:

  1. If target is in error reporting mode, then abort these steps; the error is not handled.

  2. Let target be in error reporting mode.

  3. Let message be a user-agent-defined string describing the error in a helpful manner. (This is a fingerprinting vector.)

  4. Let errorValue be the value that represents the error: in the case of an uncaught exception, that would be the value that was thrown; in the case of a JavaScript error that would be an Error object. If there is no corresponding value, then the null value must be used instead.

  5. Let urlString be the result of applying the URL serializer to the URL record that corresponds to the resource from which script was obtained.

    The resource containing the script will typically be the file from which the Document was parsed, e.g. for inline script elements or event handler content attributes; or the JavaScript file that the script was in, for external scripts. Even for dynamically-generated scripts, user agents are strongly encouraged to attempt to keep track of the original source of a script. For example, if an external script uses the document.write() API to insert an inline script element during parsing, the URL of the resource containing the script would ideally be reported as being the external script, and the line number might ideally be reported as the line with the document.write() call or where the string passed to that call was first constructed. Naturally, implementing this can be somewhat non-trivial.

    User agents are similarly encouraged to keep careful track of the original line numbers, even in the face of document.write() calls mutating the document as it is parsed, or event handler content attributes spanning multiple lines.

  6. If script has muted errors, then set message to "Script error.", urlString to the empty string, line and col to 0, and errorValue to null.

  7. Let notHandled be the result of firing an event named error at target, using ErrorEvent, with the cancelable attribute initialized to true, the message attribute initialized to message, the filename attribute initialized to urlString, the lineno attribute initialized to line, the colno attribute initialized to col, and the error attribute initialized to errorValue.

  8. Let target no longer be in error reporting mode.

  9. If notHandled is false, then the error is handled. Otherwise, the error is not handled.

    Returning true in an event handler cancels the event per the event handler processing algorithm.

8.1.3.11.1 Runtime script errors in documents

When the user agent is to report an exception E, the user agent must report the error for the relevant script, with the problematic position (line number and column number) in the resource containing the script, using the global object specified by the script's settings object as the target. If the error is still not handled after this, then the error may be reported to a developer console.

8.1.3.11.2 The ErrorEvent interface
[Constructor(DOMString type, optional ErrorEventInit eventInitDict), Exposed=(Window,Worker)]
interface ErrorEvent : Event {
  readonly attribute DOMString message;
  readonly attribute USVString filename;
  readonly attribute unsigned long lineno;
  readonly attribute unsigned long colno;
  readonly attribute any error;
};

dictionary ErrorEventInit : EventInit {
  DOMString message = "";
  USVString filename = "";
  unsigned long lineno = 0;
  unsigned long colno = 0;
  any error = null;
};

The message attribute must return the value it was initialized to. It represents the error message.

The filename attribute must return the value it was initialized to. It represents the URL of the script in which the error originally occurred.

The lineno attribute must return the value it was initialized to. It represents the line number where the error occurred in the script.

The colno attribute must return the value it was initialized to. It represents the column number where the error occurred in the script.

The error attribute must return the value it was initialized to. Where appropriate, it is set to the object representing the error (e.g., the exception object in the case of an uncaught DOM exception).

8.1.3.12 Unhandled promise rejections

In addition to synchronous runtime script errors, scripts may experience asynchronous promise rejections, tracked via the unhandledrejection and rejectionhandled events.

When the user agent is to notify about rejected promises on a given environment settings object settings object, it must run these steps:

  1. Let list be a copy of settings object's about-to-be-notified rejected promises list.

  2. If list is empty, abort these steps.

  3. Clear settings object's about-to-be-notified rejected promises list.

  4. Queue a task to run the following substep:

    1. For each promise p in list:

      1. If p's [[PromiseIsHandled]] internal slot is true, continue to the next iteration of the loop.

      2. Let notHandled be the result of firing an event named unhandledrejection at settings object's global object, using PromiseRejectionEvent, with the cancelable attribute initialized to true, the promise attribute initialized to p, and the reason attribute initialized to the value of p's [[PromiseResult]] internal slot.

      3. If notHandled is false, then the promise rejection is handled. Otherwise, the promise rejection is not handled.

      4. If p's [[PromiseIsHandled]] internal slot is false, add p to settings object's outstanding rejected promises weak set.

This algorithm results in promise rejections being marked as handled or not handled. These concepts parallel handled and not handled script errors. If a rejection is still not handled after this, then the rejection may be reported to a developer console.

8.1.3.12.1 HostPromiseRejectionTracker(promise, operation)

JavaScript contains an implementation-defined HostPromiseRejectionTracker(promise, operation) abstract operation. User agents must use the following implementation: [JAVASCRIPT]

  1. Let script be the running script.

  2. If script has muted errors, terminate these steps.

  3. Let settings object be script's settings object.

  4. If operation is "reject",

    1. Add promise to settings object's about-to-be-notified rejected promises list.

  5. If operation is "handle",

    1. If settings object's about-to-be-notified rejected promises list contains promise, remove promise from that list and abort these steps.

    2. If settings object's outstanding rejected promises weak set does not contain promise, abort these steps.

    3. Remove promise from settings object's outstanding rejected promises weak set.

    4. Queue a task to fire an event named rejectionhandled at settings object's global object, using PromiseRejectionEvent, with the promise attribute initialized to promise, and the reason attribute initialized to the value of promise's [[PromiseResult]] internal slot.

8.1.3.12.2 The PromiseRejectionEvent interface
[Constructor(DOMString type, PromiseRejectionEventInit eventInitDict), Exposed=(Window,Worker)]
interface PromiseRejectionEvent : Event {
  readonly attribute Promise<any> promise;
  readonly attribute any reason;
};

dictionary PromiseRejectionEventInit : EventInit {
  required Promise<any> promise;
  any reason;
};

The promise attribute must return the value it was initialized to. It represents the promise which this notification is about.

The reason attribute must return the value it was initialized to. It represents the rejection reason for the promise.

8.1.3.13 HostEnsureCanCompileStrings(callerRealm, calleeRealm)

JavaScript contains an implementation-defined HostEnsureCanCompileStrings(callerRealm, calleeRealm) abstract operation. User agents must use the following implementation: [JAVASCRIPT]

  1. Perform ? EnsureCSPDoesNotBlockStringCompilation(callerRealm, calleeRealm). [CSP]

8.1.4 Event loops

8.1.4.1 Definitions

To coordinate events, user interaction, scripts, rendering, networking, and so forth, user agents must use event loops as described in this section. There are two kinds of event loops: those for browsing contexts, and those for workers.

There must be at least one browsing context event loop per user agent, and at most one per unit of related similar-origin browsing contexts.

When there is more than one event loop for a unit of related browsing contexts, complications arise when a browsing context in that group is navigated such that it switches from one unit of related similar-origin browsing contexts to another. This specification does not currently describe how to handle these complications.

A browsing context event loop always has at least one browsing context. If such an event loop's browsing contexts all go away, then the event loop goes away as well. A browsing context always has an event loop coordinating its activities.

Worker event loops are simpler: each worker has one event loop, and the worker processing model manages the event loop's lifetime.


An event loop has one or more task queues. A task queue is an ordered list of tasks, which are algorithms that are responsible for such work as:

Events

Dispatching an Event object at a particular EventTarget object is often done by a dedicated task.

Not all events are dispatched using the task queue, many are dispatched during other tasks.

Parsing

The HTML parser tokenizing one or more bytes, and then processing any resulting tokens, is typically a task.

Callbacks

Calling a callback is often done by a dedicated task.

Using a resource

When an algorithm fetches a resource, if the fetching occurs in a non-blocking fashion then the processing of the resource once some or all of the resource is available is performed by a task.

Reacting to DOM manipulation

Some elements have tasks that trigger in response to DOM manipulation, e.g. when that element is inserted into the document.

Each task in a browsing context event loop is associated with a Document; if the task was queued in the context of an element, then it is the element's node document; if the task was queued in the context of a browsing context, then it is the browsing context's active document at the time the task was queued; if the task was queued by or for a script then the document is the responsible document specified by the script's settings object.

A task is intended for a specific event loop: the event loop that is handling tasks for the task's associated Document or worker.

When a user agent is to queue a task, it must add the given task to one of the task queues of the relevant event loop.

Each task is defined as coming from a specific task source. All the tasks from one particular task source and destined to a particular event loop (e.g. the callbacks generated by timers of a Document, the events fired for mouse movements over that Document, the tasks queued for the parser of that Document) must always be added to the same task queue, but tasks from different task sources may be placed in different task queues.

For example, a user agent could have one task queue for mouse and key events (the user interaction task source), and another for everything else. The user agent could then give keyboard and mouse events preference over other tasks three quarters of the time, keeping the interface responsive but not starving other task queues, and never processing events from any one task source out of order.

Each event loop has a currently running task. Initially, this is null. It is used to handle reentrancy. Each event loop also has a performing a microtask checkpoint flag, which must initially be false. It is used to prevent reentrant invocation of the perform a microtask checkpoint algorithm.

8.1.4.2 Processing model

An event loop must continually run through the following steps for as long as it exists:

  1. Let oldestTask be the oldest task on one of the event loop's task queues, if any, ignoring, in the case of a browsing context event loop, tasks whose associated Documents are not fully active. The user agent may pick any task queue. If there is no task to select, then jump to the microtasks step below.

  2. Set the event loop's currently running task to oldestTask.

  3. Run oldestTask.

  4. Set the event loop's currently running task back to null.

  5. Remove oldestTask from its task queue.

  6. Microtasks: Perform a microtask checkpoint.

  7. Update the rendering: If this event loop is a browsing context event loop (as opposed to a worker event loop), then run the following substeps.

    1. Let now be the value that would be returned by the Performance object's now() method. [HRT]

    2. Let docs be the list of Document objects associated with the event loop in question, sorted arbitrarily except that the following conditions must be met:

      In the steps below that iterate over docs, each Document must be processed in the order it is found in the list.

    3. If there are top-level browsing contexts B that the user agent believes would not benefit from having their rendering updated at this time, then remove from docs all Document objects whose browsing context's top-level browsing context is in B.

      Whether a top-level browsing context would benefit from having its rendering updated depends on various factors, such as the update frequency. For example, if the browser is attempting to achieve a 60Hz refresh rate, then these steps are only necessary every 60th of a second (about 16.7ms). If the browser finds that a top-level browsing context is not able to sustain this rate, it might drop to a more sustainable 30Hz for that set of Documents, rather than occasionally dropping frames. (This specification does not mandate any particular model for when to update the rendering.) Similarly, if a top-level browsing context is in the background, the user agent might decide to drop that page to a much slower 4Hz, or even less.

      Another example of why a browser might skip updating the rendering is to ensure certain tasks are executed immediately after each other, with only microtask checkpoints interleaved (and without, e.g., animation frame callbacks interleaved). For example, a user agent might wish to coalesce timer callbacks together, with no intermediate rendering updates.

    4. If there are a nested browsing contexts B that the user agent believes would not benefit from having their rendering updated at this time, then remove from docs all Document objects whose browsing context is in B.

      As with top-level browsing contexts, a variety of factors can influence whether it is profitable for a browser to update the rendering of nested browsing contexts. For example, a user agent might wish to spend less resources rendering third-party content, especially if it is not currently visible to the user or if resources are constrained. In such cases, the browser could decide to update the rendering for such content infrequently or never.

    5. For each fully active Document in docs, run the resize steps for that Document, passing in now as the timestamp. [CSSOMVIEW]

    6. For each fully active Document in docs, run the scroll steps for that Document, passing in now as the timestamp. [CSSOMVIEW]

    7. For each fully active Document in docs, evaluate media queries and report changes for that Document, passing in now as the timestamp. [CSSOMVIEW]

    8. For each fully active Document in docs, run CSS animations and send events for that Document, passing in now as the timestamp. [CSSANIMATIONS]

    9. For each fully active Document in docs, run the fullscreen steps for that Document, passing in now as the timestamp. [FULLSCREEN]

    10. For each fully active Document in docs, run the animation frame callbacks for that Document, passing in now as the timestamp.

    11. For each fully active Document in docs, run the update intersection observations steps for that Document, passing in now as the timestamp. [INTERSECTIONOBSERVER]

    12. For each fully active Document in docs, update the rendering or user interface of that Document and its browsing context to reflect the current state.

  8. If this is a worker event loop (i.e. one running for a WorkerGlobalScope), but there are no tasks in the event loop's task queues and the WorkerGlobalScope object's closing flag is true, then destroy the event loop, aborting these steps, resuming the run a worker steps described in the Web workers section below.


Each event loop has a microtask queue. A microtask is a task that is originally to be queued on the microtask queue rather than a task queue. There are two kinds of microtasks: solitary callback microtasks, and compound microtasks.

This specification only has solitary callback microtasks. Specifications that use compound microtasks have to take extra care to wrap callbacks to handle spinning the event loop.

When an algorithm requires a microtask to be queued, it must be appended to the relevant event loop's microtask queue; the task source of such a microtask is the microtask task source.

It is possible for a microtask to be moved to a regular task queue, if, during its initial execution, it spins the event loop. In that case, the microtask task source is the task source used. Normally, the task source of a microtask is irrelevant.

When a user agent is to perform a microtask checkpoint, if the performing a microtask checkpoint flag is false, then the user agent must run the following steps:

  1. Set the performing a microtask checkpoint flag to true.

  2. While the event loop's microtask queue is not empty:

    1. Let oldestMicrotask be the oldest microtask on the event loop's microtask queue.

    2. Set the event loop's currently running task to oldestMicrotask.

    3. Run oldestMicrotask.

      This might involve invoking scripted callbacks, which eventually calls the clean up after running script steps, which call this perform a microtask checkpoint algorithm again, which is why we use the performing a microtask checkpoint flag to avoid reentrancy.

    4. Set the event loop's currently running task back to null.

    5. Remove oldestMicrotask from the microtask queue.

  3. For each environment settings object whose responsible event loop is this event loop, notify about rejected promises on that environment settings object.

  4. Cleanup Indexed Database transactions.

  5. Set the performing a microtask checkpoint flag to false.

If, while a compound microtask is running, the user agent is required to execute a compound microtask subtask to run a series of steps, the user agent must run the following steps:

  1. Let parent be the event loop's currently running task (the currently running compound microtask).

  2. Let subtask be a new task that consists of running the given series of steps. The task source of such a microtask is the microtask task source. This is a compound microtask subtask.

  3. Set the event loop's currently running task to subtask.

  4. Run subtask.

  5. Set the event loop's currently running task back to parent.


When an algorithm running in parallel is to await a stable state, the user agent must queue a microtask that runs the following steps, and must then stop executing (execution of the algorithm resumes when the microtask is run, as described in the following steps):

  1. Run the algorithm's synchronous section.

  2. Resumes execution of the algorithm in parallel, if appropriate, as described in the algorithm's steps.

Steps in synchronous sections are marked with ⌛.


When an algorithm says to spin the event loop until a condition goal is met, the user agent must run the following steps:

  1. Let task be the event loop's currently running task.

    This might be a microtask, in which case it is a solitary callback microtask. It could also be a compound microtask subtask, or a regular task that is not a microtask. It will not be a compound microtask.

  2. Let task source be task's task source.

  3. Let old stack be a copy of the JavaScript execution context stack.

  4. Empty the JavaScript execution context stack.

  5. Perform a microtask checkpoint.

  6. Stop task, allowing whatever algorithm that invoked it to resume, but continue these steps in parallel.

    This causes one of the following algorithms to continue: the event loop's main set of steps, the perform a microtask checkpoint algorithm, or the execute a compound microtask subtask algorithm.

  7. Wait until the condition goal is met.

  8. Queue a task to continue running these steps, using the task source task source. Wait until this new task runs before continuing these steps.

  9. Replace the JavaScript execution context stack with the old stack.

  10. Return to the caller.


Some of the algorithms in this specification, for historical reasons, require the user agent to pause while running a task until a condition goal is met. This means running the following steps:

  1. If necessary, update the rendering or user interface of any Document or browsing context to reflect the current state.

  2. Wait until the condition goal is met. While a user agent has a paused task, the corresponding event loop must not run further tasks, and any script in the currently running task must block. User agents should remain responsive to user input while paused, however, albeit in a reduced capacity since the event loop will not be doing anything.

Pausing is highly detrimental to the user experience, especially in scenarios where a single event loop is shared among multiple documents. User agents are encouraged to experiment with alternatives to pausing, such as spinning the event loop or even simply proceeding without any kind of suspended execution at all, insofar as it is possible to do so while preserving compatibility with existing content. This specification will happily change if a less-drastic alternative is discovered to be web-compatible.

In the interim, implementers should be aware that the variety of alternatives that user agents might experiment with can change subtle aspects of event loop behavior, including task and microtask timing. Implementations should continue experimenting even if doing so causes them to violate the exact semantics implied by the pause operation.

8.1.4.3 Generic task sources

The following task sources are used by a number of mostly unrelated features in this and other specifications.

The DOM manipulation task source

This task source is used for features that react to DOM manipulations, such as things that happen in a non-blocking fashion when an element is inserted into the document.

The user interaction task source

This task source is used for features that react to user interaction, for example keyboard or mouse input.

Events sent in response to user input (e.g. click events) must be fired using tasks queued with the user interaction task source. [UIEVENTS]

The networking task source

This task source is used for features that trigger in response to network activity.

The history traversal task source

This task source is used to queue calls to history.back() and similar APIs.

8.1.4.4 Dealing with the event loop from other specifications

Writing specifications that correctly interact with the event loop can be tricky. This is compounded by how this specification uses a concurrency-model-independent processing model, so we say things like "event loop" and "in parallel" instead of using more familiar model-specific terms like "main thread" or "on a background thread".

The overriding guideline is that any work which would otherwise block the event loop must instead be performed in parallel with it. This includes (but is not limited to):

The next complication is that, in algorithm sections that are in parallel, you must not create or manipulate objects associated to a specific JavaScript realm, global, or environment settings object. (Stated in more familiar terms, you must not directly access main-thread artifacts from a background thread.) Doing so would create data races observable to JavaScript code, since after all, your algorithm steps are running in parallel to the JavaScript code.

You can, however, manipulate specification-level data structures and values from the WHATWG Infra Standard, as those are realm-agnostic. They are never directly exposed to JavaScript without a specific conversion taking place (often via Web IDL). [INFRA] [WEBIDL]

To affect the world of observable JavaScript objects, then, you must queue a task to perform any such manipulations. This ensures your steps are properly interleaved with respect to other things happening on the event loop. Furthermore, you must choose a task source when queueing a task; this governs the relative order of your steps versus others. If you are unsure which task source to use, pick one of the generic task sources that sounds most applicable.

Most invocations of queue a task implicitly use "the relevant event loop", i.e., the one that is obvious from context. That is because it is very rare for algorithms to be invoked in contexts involving multiple event loops. (Unlike contexts involving multiple global objects, which happen all the time!) So unless you are writing a specification which, e.g., deals with manipulating workers, you can omit this argument when queueing a task.

Putting this all together, we can provide a template for a typical algorithm that needs to do work asynchronously:

  1. Do any synchronous setup work, while still on the event loop. This may include converting realm-specific JavaScript values into realm-agnostic specification-level values.

  2. Perform a set of potentially-expensive steps in parallel, operating entirely on realm-agnostic values, and producing a realm-agnostic result.

  3. Queue a task, on a specified task source, to convert the realm-agnostic result back into observable effects on the observable world of JavaScript objects on the event loop.

The following is an algorithm that "encrypts" a passed-in list of scalar value strings input, after parsing them as URLs:

  1. Let urls be an empty list.

  2. For each string of input:

    1. Let parsed be the result of parsing string relative to the current settings object.

    2. If parsed is failure, return a promise rejected with a "SyntaxError" DOMException.

    3. Let serialized be the result of applying the URL serializer to parsed.

    4. Append serialized to urls.

  3. Let realm be the current Realm Record.

  4. Let p be a new promise.

  5. Run the following steps in parallel:

    1. Let encryptedURLs be an empty list.

    2. For each url of urls:

      1. Wait 100 milliseconds, so that people think we're doing heavy-duty encryption.

      2. Let encrypted be a new JavaScript string derived from url, whose nth code unit is equal to url's nth code unit plus 13.

      3. Append encrypted to encryptedURLs.

    3. Queue a task, on the networking task source, to perform the following steps:

      1. Let array be the result of converting encryptedURLs to a JavaScript array, in realm.

      2. Resolve p with array.

  6. Return p.

Here are several things to notice about this algorithm:

(On these last two points, see also w3ctag/promises-guide#52, heycam/webidl#135, and heycam/webidl#371, where we are still mulling over the subtleties of the above promise-resolution pattern.)

Another thing to note is that, in the event this algorithm was called from a Web IDL-specified operation taking a sequence<USVString>, there was an automatic conversion from realm-specific JavaScript objects provided by the author as input, into the realm-agnostic sequence<USVString> Web IDL type, which we then treat as a list of scalar value strings. So depending on how your specification is structured, there may be other implicit steps happening on the main event loop that play a part in this whole process of getting you ready to go in parallel.

8.1.5 Events

8.1.5.1 Event handlers

Many objects can have event handlers specified. These act as non-capture event listeners for the object on which they are specified. [DOM]

An event handler has a name, which always starts with "on" and is followed by the name of the event for which it is intended.

An event handler has a value, which is either null, or is a callback object, or is an internal raw uncompiled handler. The EventHandler callback function type describes how this is exposed to scripts. Initially, an event handler's value must be set to null.

Event handlers are exposed in one of two ways.

The first way, common to all event handlers, is as an event handler IDL attribute.

The second way is as an event handler content attribute. Event handlers on HTML elements and some of the event handlers on Window objects are exposed in this way.


An event handler IDL attribute is an IDL attribute for a specific event handler. The name of the IDL attribute is the same as the name of the event handler.

Event handler IDL attributes, on setting, must set the corresponding event handler to their new value, and on getting, must return the result of getting the current value of the event handler in question.

If an event handler IDL attribute exposes an event handler of an object that doesn't exist, it must always return null on getting and must do nothing on setting.

This can happen in particular for event handler IDL attribute on body elements that do not have corresponding Window objects.

Certain event handler IDL attributes have additional requirements, in particular the onmessage attribute of MessagePort objects.


An event handler content attribute is a content attribute for a specific event handler. The name of the content attribute is the same as the name of the event handler.

Event handler content attributes, when specified, must contain valid JavaScript code which, when parsed, would match the FunctionBody production after automatic semicolon insertion.

When an event handler content attribute is set, execute the following steps:

  1. If the Should element's inline behavior be blocked by Content Security Policy? algorithm returns "Blocked" when executed upon the attribute's element, "script attribute", and the attribute's value, then abort these steps. [CSP]

  2. Set the corresponding event handler to an internal raw uncompiled handler consisting of the attribute's new value and the script location where the attribute was set to this value

When an event handler content attribute is removed, the user agent must set the corresponding event handler to null.


When an event handler H of an element or object T implementing the EventTarget interface is first set to a non-null value, the user agent must append an event listener to the list of event listeners associated with T with type set to the event handler event type corresponding to H and callback set to the result of creating a Web IDL EventListener instance representing a reference to a function of one argument that executes the steps of the event handler processing algorithm, given H and its argument. The EventListener's callback context can be arbitrary; it does not impact the steps of the event handler processing algorithm. [DOM]

The callback is emphatically not the event handler itself. Every event handler ends up registering the same callback, the algorithm defined below, which takes care of invoking the right callback, and processing the callback's return value.

This only happens the first time the event handler's value is set. Since listeners are called in the order they were registered, the order of event listeners for a particular event type will always be first the event listeners registered with addEventListener() before the first time the event handler was set to a non-null value, then the callback to which it is currently set, if any, and finally the event listeners registered with addEventListener() after the first time the event handler was set to a non-null value.

This example demonstrates the order in which event listeners are invoked. If the button in this example is clicked by the user, the page will show four alerts, with the text "ONE", "TWO", "THREE", and "FOUR" respectively.

<button id="test">Start Demo</button>
<script>
 var button = document.getElementById('test');
 button.addEventListener('click', function () { alert('ONE') }, false);
 button.setAttribute('onclick', "alert('NOT CALLED')"); // event handler listener is registered here
 button.addEventListener('click', function () { alert('THREE') }, false);
 button.onclick = function () { alert('TWO'); };
 button.addEventListener('click', function () { alert('FOUR') }, false);
</script>

The interfaces implemented by the event object do not influence whether an event handler is triggered or not.

The event handler processing algorithm for an event handler H and an Event object E is as follows:

  1. Let callback be the result of getting the current value of the event handler H.

  2. If callback is null, then abort these steps.

  3. Let special error event handling be true if E is an ErrorEvent object, E's type is error, and E's currentTarget implements the WindowOrWorkerGlobalScope mixin. Otherwise, let special error event handling be false.

  4. Process the Event object E as follows:

    If special error event handling is true

    Invoke callback with five arguments, the first one having the value of E's message attribute, the second having the value of E's filename attribute, the third having the value of E's lineno attribute, the fourth having the value of E's colno attribute, the fifth having the value of E's error attribute, and with the callback this value set to E's currentTarget. Let return value be the callback's return value. [WEBIDL]

    Otherwise

    Invoke callback with one argument, the value of which is the Event object E, with the callback this value set to E's currentTarget. Let return value be the callback's return value. [WEBIDL]

    If an exception gets thrown by the callback, end these steps and allow the exception to propagate. (It will propagate to the DOM event dispatch logic, which will then report the exception.)

  5. Process return value as follows:

    If E is a BeforeUnloadEvent object and E's type is beforeunload

    In this case, the event handler IDL attribute's type will be OnBeforeUnloadEventHandler, so return value will have been coerced into either null or a DOMString.

    If return value is not null, then:

    1. Set E's canceled flag.

    2. If E's returnValue attribute's value is the empty string, then set E's returnValue attribute's value to return value.

    If special error event handling is true

    If return value is true, then set E's canceled flag.

    Otherwise

    If return value is false, then set E's canceled flag.

    If we've gotten to this "Otherwise" clause because E's type is beforeunload but E is not a BeforeUnloadEvent object, then return value will never be false, since in such cases return value will have been coerced into either null or a DOMString.


The EventHandler callback function type represents a callback used for event handlers. It is represented in Web IDL as follows:

[TreatNonObjectAsNull]
callback EventHandlerNonNull = any (Event event);
typedef EventHandlerNonNull? EventHandler;

In JavaScript, any Function object implements this interface.

For example, the following document fragment:

<body onload="alert(this)" onclick="alert(this)">

...leads to an alert saying "[object Window]" when the document is loaded, and an alert saying "[object HTMLBodyElement]" whenever the user clicks something in the page.

The return value of the function affects whether the event is canceled or not: as described above, if the return value is false, the event is canceled.

There are two exceptions in the platform, for historical reasons:

For historical reasons, the onerror handler has different arguments:

[TreatNonObjectAsNull]
callback OnErrorEventHandlerNonNull = any ((Event or DOMString) event, optional DOMString source, optional unsigned long lineno, optional unsigned long colno, optional any error);
typedef OnErrorEventHandlerNonNull? OnErrorEventHandler;
window.onerror = (message, source, lineno, colno, error) => { … };

Similarly, the onbeforeunload handler has a different return value:

[TreatNonObjectAsNull]
callback OnBeforeUnloadEventHandlerNonNull = DOMString? (Event event);
typedef OnBeforeUnloadEventHandlerNonNull? OnBeforeUnloadEventHandler;

An internal raw uncompiled handler is a tuple with the following information:

When the user agent is to get the current value of the event handler H, it must run these steps:

  1. If H's value is an internal raw uncompiled handler, run these substeps:

    1. If H is an element's event handler, then let element be the element, and document be the element's node document.

      Otherwise, H is a Window object's event handler: let element be null, and let document be H's associated Document.

    2. If scripting is disabled for document, then return null.

    3. Let body be the uncompiled script body in the internal raw uncompiled handler.

    4. Let location be the location where the script body originated, as given by the internal raw uncompiled handler.

    5. If element is not null and element has a form owner, let form owner be that form owner. Otherwise, let form owner be null.

    6. Let settings object be the relevant settings object of document.

    7. If body is not parsable as FunctionBody or if parsing detects an early error, then follow these substeps:

      1. Set H's value to null.

      2. Report the error for the appropriate script and with the appropriate position (line number and column number) given by location, using settings object's global object. If the error is still not handled after this, then the error may be reported to a developer console.

      3. Return null.

    8. If body begins with a Directive Prologue that contains a Use Strict Directive then let strict be true, otherwise let strict be false.

    9. Push settings object's realm execution context onto the JavaScript execution context stack; it is now the running JavaScript execution context.

      This is necessary so the subsequent invocation of FunctionCreate takes place in the correct JavaScript Realm.

    10. Let function be the result of calling FunctionCreate, with arguments:

      kind
      Normal
      ParameterList
      If H is an onerror event handler of a Window object
      Let the function have five arguments, named event, source, lineno, colno, and error.
      Otherwise
      Let the function have a single argument called event.
      Body
      The result of parsing body above.
      Scope
      1. If H is an element's event handler, then let Scope be NewObjectEnvironment(document, the global environment).

        Otherwise, H is a Window object's event handler: let Scope be the global environment.

      2. If form owner is not null, let Scope be NewObjectEnvironment(form owner, Scope).

      3. If element is not null, let Scope be NewObjectEnvironment(element, Scope).

      Strict
      The value of strict.
    11. Remove settings object's realm execution context from the JavaScript execution context stack.

    12. Set H's value to the result of creating a Web IDL callback function whose object reference is function and whose callback context is settings object.

  2. Return H's value.

8.1.5.2 Event handlers on elements, Document objects, and Window objects

The following are the event handlers (and their corresponding event handler event types) that must be supported by all HTML elements, as both event handler content attributes and event handler IDL attributes; and that must be supported by all Document and Window objects, as event handler IDL attributes:

Event handler Event handler event type
onabort abort
onauxclick auxclick
oncancel cancel
oncanplay canplay
oncanplaythrough canplaythrough
onchange change
onclick click
onclose close
oncontextmenu contextmenu
oncuechange cuechange
ondblclick dblclick
ondrag drag
ondragend dragend
ondragenter dragenter
ondragexit dragexit
ondragleave dragleave
ondragover dragover
ondragstart dragstart
ondrop drop
ondurationchange durationchange
onemptied emptied
onended ended
oninput input
oninvalid invalid
onkeydown keydown
onkeypress keypress
onkeyup keyup
onloadeddata loadeddata
onloadedmetadata loadedmetadata
onloadend loadend
onloadstart loadstart
onmousedown mousedown
onmouseenter mouseenter
onmouseleave mouseleave
onmousemove mousemove
onmouseout mouseout
onmouseover mouseover
onmouseup mouseup
onwheel wheel
onpause pause
onplay play
onplaying playing
onprogress progress
onratechange ratechange
onreset reset
onseeked seeked
onseeking seeking
onselect select
onstalled stalled
onsubmit submit
onsuspend suspend
ontimeupdate timeupdate
ontoggle toggle
onvolumechange volumechange
onwaiting waiting

The following are the event handlers (and their corresponding event handler event types) that must be supported by all HTML elements other than body and frameset elements, as both event handler content attributes and event handler IDL attributes; that must be supported by all Document objects, as event handler IDL attributes; and that must be supported by all Window objects, as event handler IDL attributes on the Window objects themselves, and with corresponding event handler content attributes and event handler IDL attributes exposed on all body and frameset elements that are owned by that Window object's associated Document:

Event handler Event handler event type
onblur blur
onerror error
onfocus focus
onload load
onresize resize
onscroll scroll

The following are the event handlers (and their corresponding event handler event types) that must be supported by Window objects, as event handler IDL attributes on the Window objects themselves, and with corresponding event handler content attributes and event handler IDL attributes exposed on all body and frameset elements that are owned by that Window object's associated Document:

Event handler Event handler event type
onafterprint afterprint
onbeforeprint beforeprint
onbeforeunload beforeunload
onhashchange hashchange
onlanguagechange languagechange
onmessage message
onmessageerror messageerror
onoffline offline
ononline online
onpagehide pagehide
onpageshow pageshow
onpopstate popstate
onrejectionhandled rejectionhandled
onstorage storage
onunhandledrejection unhandledrejection
onunload unload

The following are the event handlers (and their corresponding event handler event types) that must be supported by all HTML elements, as both event handler content attributes and event handler IDL attributes; and that must be supported by all Document objects, as event handler IDL attributes:

Event handler Event handler event type
oncut cut
oncopy copy
onpaste paste

The following are the event handlers (and their corresponding event handler event types) that must be supported on Document objects as event handler IDL attributes:

Event handler Event handler event type
onreadystatechange readystatechange
8.1.5.2.1 IDL definitions
[NoInterfaceObject]
interface GlobalEventHandlers {
  attribute EventHandler onabort;
  attribute EventHandler onauxclick;
  attribute EventHandler onblur;
  attribute EventHandler oncancel;
  attribute EventHandler oncanplay;
  attribute EventHandler oncanplaythrough;
  attribute EventHandler onchange;
  attribute EventHandler onclick;
  attribute EventHandler onclose;
  attribute EventHandler oncontextmenu;
  attribute EventHandler oncuechange;
  attribute EventHandler ondblclick;
  attribute EventHandler ondrag;
  attribute EventHandler ondragend;
  attribute EventHandler ondragenter;
  attribute EventHandler ondragexit;
  attribute EventHandler ondragleave;
  attribute EventHandler ondragover;
  attribute EventHandler ondragstart;
  attribute EventHandler ondrop;
  attribute EventHandler ondurationchange;
  attribute EventHandler onemptied;
  attribute EventHandler onended;
  attribute OnErrorEventHandler onerror;
  attribute EventHandler onfocus;
  attribute EventHandler oninput;
  attribute EventHandler oninvalid;
  attribute EventHandler onkeydown;
  attribute EventHandler onkeypress;
  attribute EventHandler onkeyup;
  attribute EventHandler onload;
  attribute EventHandler onloadeddata;
  attribute EventHandler onloadedmetadata;
  attribute EventHandler onloadend;
  attribute EventHandler onloadstart;
  attribute EventHandler onmousedown;
  [LenientThis] attribute EventHandler onmouseenter;
  [LenientThis] attribute EventHandler onmouseleave;
  attribute EventHandler onmousemove;
  attribute EventHandler onmouseout;
  attribute EventHandler onmouseover;
  attribute EventHandler onmouseup;
  attribute EventHandler onwheel;
  attribute EventHandler onpause;
  attribute EventHandler onplay;
  attribute EventHandler onplaying;
  attribute EventHandler onprogress;
  attribute EventHandler onratechange;
  attribute EventHandler onreset;
  attribute EventHandler onresize;
  attribute EventHandler onscroll;
  attribute EventHandler onseeked;
  attribute EventHandler onseeking;
  attribute EventHandler onselect;
  attribute EventHandler onstalled;
  attribute EventHandler onsubmit;
  attribute EventHandler onsuspend;
  attribute EventHandler ontimeupdate;
  attribute EventHandler ontoggle;
  attribute EventHandler onvolumechange;
  attribute EventHandler onwaiting;
};

[NoInterfaceObject]
interface WindowEventHandlers {
  attribute EventHandler onafterprint;
  attribute EventHandler onbeforeprint;
  attribute OnBeforeUnloadEventHandler onbeforeunload;
  attribute EventHandler onhashchange;
  attribute EventHandler onlanguagechange;
  attribute EventHandler onmessage;
  attribute EventHandler onmessageerror;
  attribute EventHandler onoffline;
  attribute EventHandler ononline;
  attribute EventHandler onpagehide;
  attribute EventHandler onpageshow;
  attribute EventHandler onpopstate;
  attribute EventHandler onrejectionhandled;
  attribute EventHandler onstorage;
  attribute EventHandler onunhandledrejection;
  attribute EventHandler onunload;
};

[NoInterfaceObject]
interface DocumentAndElementEventHandlers {
  attribute EventHandler oncopy;
  attribute EventHandler oncut;
  attribute EventHandler onpaste;
};
8.1.5.3 Event firing

Certain operations and methods are defined as firing events on elements. For example, the click() method on the HTMLElement interface is defined as firing a click event on the element. [UIEVENTS]

Firing a synthetic mouse event named e at target, with an optional not trusted flag, means running these steps:

  1. Let event be the result of creating an event using MouseEvent.

  2. Initialize event's type attribute to e.

  3. Initialize event's bubbles and cancelable attributes to true.

  4. If the not trusted flag is set, initialize event's isTrusted attribute to false.

  5. Initialize event's ctrlKey, shiftKey, altKey, and metaKey attributes according to the current state of the key input device, if any (false for any keys that are not available).

  6. Initialize event's view attribute to target's node document's Window object, if any, and null otherwise.

  7. event's getModifierState() method is to return values appropriately describing the current state of the key input device.

  8. Return the result of dispatching event at target.

Firing a click event at target means firing a synthetic mouse event named click at target.

8.2 The WindowOrWorkerGlobalScope mixin

The WindowOrWorkerGlobalScope mixin is for use of APIs that are to be exposed on Window and WorkerGlobalScope objects.

Other standards are encouraged to further extend it using partial interface WindowOrWorkerGlobalScope { … }; along with an appropriate reference.

typedef (DOMString or Function) TimerHandler;

[NoInterfaceObject, Exposed=(Window,Worker)]
interface WindowOrWorkerGlobalScope {
  [Replaceable] readonly attribute USVString origin;

  // base64 utility methods
  DOMString btoa(DOMString data);
  DOMString atob(DOMString data);

  // timers
  long setTimeout(TimerHandler handler, optional long timeout = 0, any... arguments);
  void clearTimeout(optional long handle = 0);
  long setInterval(TimerHandler handler, optional long timeout = 0, any... arguments);
  void clearInterval(optional long handle = 0);

  // ImageBitmap
  Promise<ImageBitmap> createImageBitmap(ImageBitmapSource image, optional ImageBitmapOptions options);
  Promise<ImageBitmap> createImageBitmap(ImageBitmapSource image, long sx, long sy, long sw, long sh, optional ImageBitmapOptions options);
};
Window implements WindowOrWorkerGlobalScope;
WorkerGlobalScope implements WindowOrWorkerGlobalScope;
origin = self . origin

Returns the global object's origin, serialized as string.

Developers are strongly encouraged to use self.origin over location.origin. The former returns the origin of the environment, the latter of the URL of the environment. Imagine the following script executing in a document on https://stargate.example/:

var frame = document.createElement("iframe")
frame.onload = function() {
  var frameWin = frame.contentWindow
  console.log(frameWin.location.origin) // "null"
  console.log(frameWin.origin) // "https://stargate.example"
}
document.body.appendChild(frame)

self.origin is a more reliable security indicator.

The origin attribute's getter must return this object's relevant settings object's origin, serialized.

8.3 Base64 utility methods

Support: atob-btoaChrome for Android 59+Chrome 4+iOS Safari 3.2+UC Browser for Android 11.4+Firefox 2+IE 10+Samsung Internet 4+Opera Mini all+Safari 3.1+Android Browser 2.1+Edge 12+Opera 10.6+

Source: caniuse.com

The atob() and btoa() methods allow developers to transform content to and from the base64 encoding.

In these APIs, for mnemonic purposes, the "b" can be considered to stand for "binary", and the "a" for "ASCII". In practice, though, for primarily historical reasons, both the input and output of these functions are Unicode strings.

result = self . btoa( data )

Takes the input data, in the form of a Unicode string containing only characters in the range U+0000 to U+00FF, each representing a binary byte with values 0x00 to 0xFF respectively, and converts it to its base64 representation, which it returns.

Throws an "InvalidCharacterError" DOMException exception if the input string contains any out-of-range characters.

result = self . atob( data )

Takes the input data, in the form of a Unicode string containing base64-encoded binary data, decodes it, and returns a string consisting of characters in the range U+0000 to U+00FF, each representing a binary byte with values 0x00 to 0xFF respectively, corresponding to that binary data.

Throws an "InvalidCharacterError" DOMException if the input string is not valid base64 data.

The btoa(data) method must throw an "InvalidCharacterError" DOMException if data contains any character whose code point is greater than U+00FF. Otherwise, the user agent must convert data to a sequence of octets whose nth octet is the eight-bit representation of the code point of the nth character of data, and then must apply the base64 algorithm to that sequence of octets, and return the result. [RFC4648]

The atob(data) method, when invoked, must run the following steps:

  1. Let position be a pointer into data, initially pointing at the start of the string.

  2. Remove all ASCII whitespace from data.

  3. If the length of data divides by 4 leaving no remainder, then: if data ends with one or two U+003D EQUALS SIGN (=) characters, remove them from data.

  4. If the length of data divides by 4 leaving a remainder of 1, throw an "InvalidCharacterError" DOMException and abort these steps.

  5. If data contains a character that is not in the following list of characters and character ranges, throw an "InvalidCharacterError" DOMException and abort these steps:

  6. Let output be a string, initially empty.

  7. Let buffer be a buffer that can have bits appended to it, initially empty.

  8. While position does not point past the end of data, run these substeps:

    1. Find the character pointed to by position in the first column of the following table. Let n be the number given in the second cell of the same row.

      Character Number
      A0
      B1
      C2
      D3
      E4
      F5
      G6
      H7
      I8
      J9
      K10
      L11
      M12
      N13
      O14
      P15
      Q16
      R17
      S18
      T19
      U20
      V21
      W22
      X23
      Y24
      Z25
      a26
      b27
      c28
      d29
      e30
      f31
      g32
      h33
      i34
      j35
      k36
      l37
      m38
      n39
      o40
      p41
      q42
      r43
      s44
      t45
      u46
      v47
      w48
      x49
      y50
      z51
      052
      153
      254
      355
      456
      557
      658
      759
      860
      961
      +62
      /63
    2. Append to buffer the six bits corresponding to number, most significant bit first.

    3. If buffer has accumulated 24 bits, interpret them as three 8-bit big-endian numbers. Append the three characters with code points equal to those numbers to output, in the same order, and then empty buffer.

    4. Advance position by one character.

  9. If buffer is not empty, it contains either 12 or 18 bits. If it contains 12 bits, discard the last four and interpret the remaining eight as an 8-bit big-endian number. If it contains 18 bits, discard the last two and interpret the remaining 16 as two 8-bit big-endian numbers. Append the one or two characters with code points equal to those one or two numbers to output, in the same order.

    The discarded bits mean that, for instance, atob("YQ") and atob("YR") both return "a".

  10. Return output.