1. 8 Web application APIs
    1. 8.1 Scripting
      1. 8.1.1 Introduction
      2. 8.1.2 Agents and agent clusters
        1. 8.1.2.1 Integration with the JavaScript agent formalism
        2. 8.1.2.2 Integration with the JavaScript agent cluster formalism
      3. 8.1.3 Script processing model
        1. 8.1.3.1 Runtime script errors
        2. 8.1.3.2 Unhandled promise rejections
      4. 8.1.4 Module specifier resolution
        1. 8.1.4.1 The resolution algorithm
        2. 8.1.4.2 Import maps
        3. 8.1.4.3 Module-related host hooks
      5. 8.1.5 Event loops
      6. 8.1.6 Events
        1. 8.1.6.1 Event handlers
        2. 8.1.6.2 Event handlers on elements, Document objects, and Window objects
    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 Agents and agent clusters

8.1.2.1 Integration with the JavaScript agent formalism

JavaScript defines the concept of an agent. This section gives the mapping of that language-level concept on to the web platform.

Conceptually, the agent concept is an architecture-independent, idealized "thread" in which JavaScript code runs. Such code can involve multiple globals/realms that can synchronously access each other, and thus needs to run in a single execution thread.

The following types of agents exist on the web platform:

Similar-origin window agent

Contains various Window objects which can potentially reach each other, either directly or by using document.domain.

If the encompassing agent cluster's is origin-keyed is true, then all the Window objects will be same origin, can reach each other directly, and document.domain will no-op.

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

Dedicated worker agent

Contains a single DedicatedWorkerGlobalScope.

Shared worker agent

Contains a single SharedWorkerGlobalScope.

Service worker agent

Contains a single ServiceWorkerGlobalScope.

Worklet agent

Contains a single WorkletGlobalScope object.

Although a given worklet can have multiple realms, each such realm needs its own agent, as each realm can be executing code independently and at the same time as the others.

Only shared and dedicated worker agents allow the use of JavaScript Atomics APIs to potentially block.

8.1.2.2 Integration with the JavaScript agent cluster formalism

JavaScript also defines the concept of an agent cluster, which this standard maps to the web platform by placing agents appropriately when they are created.

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.

Conceptually, the agent cluster concept is an architecture-independent, idealized "process boundary" that groups together multiple "threads" (agents). The agent clusters defined by the specification are generally more restrictive than the actual process boundaries implemented in user agents. By enforcing these idealized divisions at the specification level, we ensure that web developers see interoperable behavior with regard to shared memory, even in the face of varying and changing user agent process models.

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 Script processing model

8.1.3.1 Runtime script errors

reportError

Support in all current engines.

Firefox93+Safari15.4+Chrome95+
Opera?Edge95+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?
self.reportError(e)

Dispatches an error event at the global object for the given value e, in the same fashion as an unhandled exception.

In various scenarios, the user agent can report an exception by firing an error event at the Window. If this event is not canceled, then the error is considered not handled, and can be reported to the developer console.

8.1.3.2 Unhandled promise rejections

Window/rejectionhandled_event

Support in all current engines.

Firefox69+Safari11+Chrome49+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS11.3+Chrome Android?WebView Android?Samsung Internet?Opera Android?

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

8.1.4 Module specifier resolution

8.1.4.1 The resolution algorithm

The resolve a module specifier algorithm is the primary entry point for converting module specifier strings into URLs. When no import maps are involved, it is relatively straightforward, and reduces to resolving a URL-like module specifier.

When there is a non-empty import map present, the behavior is more complex. It checks candidate entries from all applicable module specifier maps, from most-specific to least-specific scopes (falling back to the top-level unscoped imports), and from most-specific to least-specific prefixes.

In the end, if no successful resolution is found via any of the candidate module specifier maps, resolve a module specifier will throw an exception. Thus the result is always either a URL or a thrown exception.

8.1.4.2 Import maps

An import map allows control over module specifier resolution. Import maps are delivered via inline script elements with their type attribute set to "importmap", and with their child text content containing a JSON representation of the import map.

Only one import map is processed per Document. After the first import map is seen, others will be ignored, with their corresponding script elements generating error events. Similarly, once any modules have been imported, e.g., via import() expressions or script elements with their type attribute set to "module", further import maps will be ignored.

These restrictions, as well as the lack of support for external import maps, are in place to keep the initial version of the feature simple. They might be lifted over time as implementer bandwidth allows.

The simplest use of import maps is to globally remap a bare module specifier:

{
  "imports": {
    "moment": "/node_modules/moment/src/moment.js"
  }
}

This enables statements like import moment from "moment"; to work, fetching and evaluating the JavaScript module at the /node_modules/moment/src/moment.js URL.

An import map can remap a class of module specifiers into a class of URLs by using trailing slashes, like so:

{
  "imports": {
    "moment/": "/node_modules/moment/src/"
  }
}

This enables statements like import localeData from "moment/locale/zh-cn.js"; to work, fetching and evaluating the JavaScript module at the /node_modules/moment/src/locale/zh-cn.js URL. Such trailing-slash mappings are often combined with bare-specifier mappings, e.g.

{
  "imports": {
    "moment": "/node_modules/moment/src/moment.js",
    "moment/": "/node_modules/moment/src/"
  }
}

so that both the "main module" specified by "moment" and the "submodules" specified by paths such as "moment/locale/zh-cn.js" are available.

Bare specifiers are not the only type of module specifiers which import maps can remap. "URL-like" specifiers, i.e., those that are either parseable as absolute URLs or start with "/", "./", or "../", can be remapped as well:

{
  "imports": {
    "https://cdn.example.com/vue/dist/vue.runtime.esm.js": "/node_modules/vue/dist/vue.runtime.esm.js",
    "/js/app.mjs": "/js/app-8e0d62a03.mjs",
    "../helpers/": "https://cdn.example/helpers/"
  }
}

Note how the URL to be remapped, as well as the URL being mapped to, can be specified either as absolute URLs, or as relative URLs starting with "/", "./", or "../". (They cannot be specified as relative URLs without those starting sigils, as those help distinguish from bare module specifiers.) Also note how the trailing slash mapping works in this context as well.

Such remappings operate on the post-canonicalization URL, and do not require a match between the literal strings supplied in the import map key and the imported module specifier. So for example, if this import map was included on https://example.com/app.html, then not only would import "/js/app.mjs" be remapped, but so would import "./js/app.mjs" and import "./foo/../js/app.mjs".

All previous examples have globally remapped module specifiers, by using the top-level "imports" key in the import map. The top-level "scopes" key can be used to provide localized remappings, which only apply when the referring module matches a specific URL prefix. For example:

{
  "scopes": {
    "/a/" : {
      "moment": "/node_modules/moment/src/moment.js"
    },
    "/b/" : {
      "moment": "https://cdn.example.com/moment/src/moment.js"
    }
  }
}

With this import map, the statement import "moment" will have different meanings depending on which referrer script contains the statement:

A typical usage of scopes is to allow multiple versions of the "same" module to exist in a web application, with some parts of the module graph importing one version, and other parts importing another version.

Scopes can overlap each other, and overlap the global "imports" specifier map. At resolution time, scopes are consulted in order of most- to least-specific, where specificity is measured by sorting the scopes using the code unit less than operation. So, for example, "/scope2/scope3/" is treated as more specific than "/scope2/", which is treated as more specific than the top-level (unscoped) mappings.

The following import map illustrates this:

{
  "imports": {
    "a": "/a-1.mjs",
    "b": "/b-1.mjs",
    "c": "/c-1.mjs"
  },
  "scopes": {
    "/scope2/": {
      "a": "/a-2.mjs"
    },
    "/scope2/scope3/": {
      "b": "/b-3.mjs"
    }
  }
}

This results in the following resolutions (using relative URLs for brevity):

Specifier
"a" "b" "c"
Referrer /scope1/r.mjs /a-1.mjs /b-1.mjs /c-1.mjs
/scope2/r.mjs /a-2.mjs /b-1.mjs /c-1.mjs
/scope2/scope3/r.mjs /a-2.mjs /b-3.mjs /c-1.mjs

Import maps can also be used to provide modules with integrity metadata to be used in Subresource Integrity checks. [SRI]

The following import map illustrates this:

{
  "imports": {
    "a": "/a-1.mjs",
    "b": "/b-1.mjs",
    "c": "/c-1.mjs"
  },
  "integrity": {
    "/a-1.mjs": "sha384-Li9vy3DqF8tnTXuiaAJuML3ky+er10rcgNR/VqsVpcw+ThHmYcwiB1pbOxEbzJr7",
    "/d-1.mjs": "sha384-MBO5IDfYaE6c6Aao94oZrIOiC6CGiSN2n4QUbHNPhzk5Xhm0djZLQqTpL0HzTUxk"
  }
}

The above example provides integrity metadata to be enforced on the modules /a-1.mjs and /d-1.mjs, even if the latter is not defined as an import in the map.


The child text content of a script element representing an import map must match the following import map authoring requirements:

A valid module specifier map is a JSON object that meets the following requirements:

8.1.4.3 Module-related host hooks

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.

modulePromise = import(specifier)

Returns a promise for the module namespace object for the module script identified by specifier. This allows dynamic importing of module scripts at runtime, instead of statically using the import statement form. The specifier will be resolved relative to the active script.

The returned promise will be rejected if an invalid specifier is given, or if a failure is encountered while fetching or evaluating the resulting module graph.

This syntax can be used inside both classic and module scripts. It thus provides a bridge into the module-script world, from the classic-script world.

url = import.meta.url

Returns the active module script's base URL.

This syntax can only be used inside module scripts.

url = import.meta.resolve(specifier)

Returns specifier, resolved relative to the active script. That is, this returns the URL that would be imported by using import(specifier).

Throws a TypeError exception if an invalid specifier is given.

This syntax can only be used inside module scripts.

Module maps are used to ensure that imported module scripts are only fetched, parsed, and evaluated once per Document or worker.

Since module maps are keyed by (URL, module type), the following code will create three separate entries in the module map, since it results in three different (URL, module type) tuples (all with "javascript-or-wasm" type):

import "https://example.com/module.mjs";
import "https://example.com/module.mjs#map-buster";
import "https://example.com/module.mjs?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.mjs";
import "https:example.com/module2.mjs";
import "https://///example.com\\module2.mjs";
import "https://example.com/foo/../module2.mjs";

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.

Since module type is also part of the module map key, the following code will create two separate entries in the module map (the type is "javascript-or-wasm" for the first, and "css" for the second):

<script type=module>
  import "https://example.com/module";
</script>
<script type=module>
  import "https://example.com/module" with { type: "css" };
</script>

This can result in two separate fetches and two separate module evaluations being performed.

In practice, due to the as-yet-unspecified memory cache (see issue #6110) the resource may only be fetched once in WebKit and Blink-based browsers. Additionally, as long as all module types are mutually exclusive, the module type check in fetch a single module script will fail for at least one of the imports, so at most one module evaluation will occur.

The purpose of including the type in the module map key is so that an import with the wrong type attribute does not prevent a different import of the same specifier but with the correct type from succeeding.

JavaScript module scripts are the default import type when importing from another JavaScript module; that is, when an import statement lacks a type import attribute the imported module script's type will be JavaScript. Attempting to import a JavaScript resource using an import statement with a type import attribute will fail:

<script type="module">
    // All of the following will fail, assuming that the imported .mjs files are served with a
    // JavaScript MIME type. JavaScript module scripts are the default and cannot be imported with
    // any import type attribute.
    import foo from "./foo.mjs" with { type: "javascript" };
    import foo2 from "./foo2.mjs" with { type: "js" };
    import foo3 from "./foo3.mjs" with { type: "" };
    await import("./foo4.mjs", { with: { type: null } });
    await import("./foo5.mjs", { with: { type: undefined } });
</script>

8.1.5 Event loops

To coordinate events, user interaction, scripts, rendering, networking, and so forth, user agents must use event loops as described in this section. Each agent has an associated event loop, which is unique to that agent.

The event loop of a similar-origin window agent is known as a window event loop. The event loop of a dedicated worker agent, shared worker agent, or service worker agent is known as a worker event loop. And the event loop of a worklet agent is known as a worklet event loop.

Event loops do not necessarily correspond to implementation threads. For example, multiple window event loops could be cooperatively scheduled in a single thread.

However, for the various worker agents that are allocated with [[CanBlock]] set to true, the JavaScript specification does place requirements on them regarding forward progress, which effectively amount to requiring dedicated per-agent threads in those cases.

8.1.6 Events

8.1.6.1 Event handlers

Events/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]

Event handlers are exposed in 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.

For both of these two ways, the event handler is exposed through a name, which is a string that always starts with "on" and is followed by the name of the event for which the handler is intended.


Most of the time, the object that exposes an event handler is the same as the object on which the corresponding event listener is added. However, the body and frameset elements expose several event handlers that act upon the element's Window object, if one exists. In either case, we call the object an event handler acts upon the target of that event handler.


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.


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.

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>

However, in the following example, the event handler is deactivated after its initial activation (and its event listener is removed), before being reactivated at a later time. The page will show five alerts with "ONE", "TWO", "THREE", "FOUR", and "FIVE" respectively, in order.

<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 is activated here
 button.addEventListener('click', function () { alert('TWO') }, false);
 button.onclick = null;                                 // but deactivated here
 button.addEventListener('click', function () { alert('THREE') }, false);
 button.onclick = function () { alert('FOUR'); };       // and re-activated here
 button.addEventListener('click', function () { alert('FIVE') }, false);
</script>

The EventHandler callback function type represents a callback used for event handlers.

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: 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:

window.onerror = (message, source, lineno, colno, error) => {};

Similarly, the onbeforeunload handler has a different return value: it will be cast to a string.

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

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

Event handler Event handler event type
onabort

HTMLMediaElement/abort_event

Support in all current engines.

Firefox9+Safari3.1+Chrome3+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+
abort
onauxclick

Element/auxclick_event

Firefox53+SafariNoChrome55+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android53+Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?
auxclick
onbeforeinput beforeinput
onbeforematch beforematch
onbeforetoggle beforetoggle
oncancel

HTMLDialogElement/cancel_event

Support in all current engines.

Firefox98+Safari15.4+Chrome37+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome AndroidNoWebView Android?Samsung Internet?Opera Android?
cancel
oncanplay

HTMLMediaElement/canplay_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
canplay
oncanplaythrough

HTMLMediaElement/canplaythrough_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
canplaythrough
onchange

HTMLElement/change_event

Support in all current engines.

Firefox1+Safari3+Chrome1+
Opera9+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android10.1+
change
onclick

Element/click_event

Support in all current engines.

Firefox6+Safari3+Chrome1+
Opera11.6+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android6+Safari iOS1+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+
click
onclose close
oncontextlost contextlost
oncontextmenu contextmenu
oncontextrestored contextrestored
oncopy

Element/copy_event

Support in all current engines.

Firefox22+Safari3+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+
copy
oncuechange

HTMLTrackElement/cuechange_event

Support in all current engines.

Firefox68+Safari10+Chrome32+
Opera19+Edge79+
Edge (Legacy)14+Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android4.4.3+Samsung Internet?Opera Android19+
cuechange
oncut

Element/cut_event

Support in all current engines.

Firefox22+Safari3+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+
cut
ondblclick

Element/dblclick_event

Support in all current engines.

Firefox6+Safari3+Chrome1+
Opera11.6+Edge79+
Edge (Legacy)12+Internet Explorer8+
Firefox Android6+Safari iOS1+Chrome AndroidNoWebView Android?Samsung Internet?Opera Android12.1+
dblclick
ondrag drag
ondragend dragend
ondragenter dragenter
ondragleave dragleave
ondragover dragover
ondragstart dragstart
ondrop drop
ondurationchange

HTMLMediaElement/durationchange_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
durationchange
onemptied

HTMLMediaElement/emptied_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
emptied
onended

HTMLMediaElement/ended_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
ended
onformdata formdata
oninput

HTMLElement/input_event

Support in all current engines.

Firefox6+Safari3.1+Chrome1+
Opera11.6+Edge79+
Edge (Legacy)NoInternet Explorer🔰 9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12+
input
oninvalid invalid
onkeydown

Element/keydown_event

Support in all current engines.

Firefox6+Safari1.2+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+
keydown
onkeypress keypress
onkeyup

Element/keyup_event

Support in all current engines.

Firefox6+Safari1.2+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+
keyup
onloadeddata

HTMLMediaElement/loadeddata_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
loadeddata
onloadedmetadata

HTMLMediaElement/loadedmetadata_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
loadedmetadata
onloadstart

HTMLMediaElement/loadstart_event

Support in all current engines.

Firefox6+Safari4+Chrome3+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+
loadstart
onmousedown

Element/mousedown_event

Support in all current engines.

Firefox6+Safari4+Chrome2+
Opera11.6+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+
mousedown
onmouseenter

Element/mouseenter_event

Support in all current engines.

Firefox10+Safari7+Chrome30+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer5.5+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android?
mouseenter
onmouseleave

Element/mouseleave_event

Support in all current engines.

Firefox10+Safari7+Chrome30+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer5.5+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android?
mouseleave
onmousemove

Element/mousemove_event

Support in all current engines.

Firefox6+Safari4+Chrome2+
Opera11.6+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+
mousemove
onmouseout

Element/mouseout_event

Support in all current engines.

Firefox6+Safari1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+
mouseout
onmouseover

Element/mouseover_event

Support in all current engines.

Firefox6+Safari4+Chrome2+
Opera9.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android10.1+
mouseover
onmouseup

Element/mouseup_event

Support in all current engines.

Firefox6+Safari4+Chrome2+
Opera11.6+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+
mouseup
onpaste

Element/paste_event

Support in all current engines.

Firefox22+Safari3+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+
paste
onpause

HTMLMediaElement/pause_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
pause
onplay

HTMLMediaElement/play_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
play
onplaying

HTMLMediaElement/playing_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
playing
onprogress

HTMLMediaElement/progress_event

Support in all current engines.

Firefox6+Safari3.1+Chrome3+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+
progress
onratechange

HTMLMediaElement/ratechange_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
ratechange
onreset reset
onscrollend

Document/scrollend_event

Firefox109+SafariNoChrome114+
Opera?Edge114+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

Element/scrollend_event

Firefox109+SafariNoChrome114+
Opera?Edge114+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?
scrollend
onsecuritypolicyviolation

Element/securitypolicyviolation_event

Support in all current engines.

Firefox63+Safari10+Chrome41+
Opera?Edge79+
Edge (Legacy)15+Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?
securitypolicyviolation
onseeked

HTMLMediaElement/seeked_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
seeked
onseeking

HTMLMediaElement/seeking_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
seeking
onselect

HTMLInputElement/select_event

Support in all current engines.

Firefox6+Safari1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+

HTMLTextAreaElement/select_event

Support in all current engines.

Firefox6+Safari1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+
select
onslotchange

HTMLSlotElement/slotchange_event

Support in all current engines.

Firefox63+Safari10.1+Chrome53+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?
slotchange
onstalled

HTMLMediaElement/stalled_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
stalled
onsubmit

HTMLFormElement/submit_event

Support in all current engines.

Firefox1+Safari3+Chrome1+
Opera8+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS1+Chrome Android?WebView Android?Samsung Internet?Opera Android10.1+
submit
onsuspend

HTMLMediaElement/suspend_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
suspend
ontimeupdate

HTMLMediaElement/timeupdate_event

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+
Opera10.5+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
timeupdate
ontoggle toggle
onvolumechange

HTMLMediaElement/volumechange_event

Support in all current engines.

Firefox6+Safari3.1+Chrome3+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+
volumechange
onwaiting

HTMLMediaElement/waiting_event

Support in all current engines.

Firefox6+Safari3.1+Chrome3+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+
waiting
onwebkitanimationend webkitAnimationEnd
onwebkitanimationiteration webkitAnimationIteration
onwebkitanimationstart webkitAnimationStart
onwebkittransitionend webkitTransitionEnd
onwheel

Element/wheel_event

Support in all current engines.

Firefox17+Safari7+Chrome31+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOSNoChrome Android?WebView Android?Samsung Internet?Opera Android?
wheel

The following are the event handlers (and their corresponding event handler event types) supported by all HTML elements other than body and frameset elements, as both event handler content attributes and event handler IDL attributes; supported by all Document objects, as event handler IDL attributes; and 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

Element/blur_event

Support in all current engines.

Firefox24+Safari3.1+Chrome1+
Opera11.6+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+

Window/blur_event

Support in all current engines.

Firefox6+Safari5.1+Chrome5+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer11
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+
blur
onerror

Window/error_event

Support in all current engines.

Firefox6+Safari5.1+Chrome10+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android?
error
onfocus

Element/focus_event

Support in all current engines.

Firefox24+Safari3.1+Chrome1+
Opera11.6+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+

Window/focus_event

Support in all current engines.

Firefox6+Safari5.1+Chrome5+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer11
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+
focus
onload load
onresize resize
onscroll

Document/scroll_event

Support in all current engines.

Firefox6+Safari2+Chrome1+
Opera11.6+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12+

Element/scroll_event

Support in all current engines.

Firefox6+Safari1.3+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+
scroll

We call the set of the names of the event handlers listed in the first column of this table the Window-reflecting body element event handler set.


The following are the event handlers (and their corresponding event handler event types) 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

Window/afterprint_event

Support in all current engines.

Firefox6+Safari13+Chrome63+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?
afterprint
onbeforeprint

Window/beforeprint_event

Support in all current engines.

Firefox6+Safari13+Chrome63+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?
beforeprint
onbeforeunload

Window/beforeunload_event

Support in all current engines.

Firefox1+Safari3+Chrome1+
Opera12+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS1+Chrome Android?WebView Android?Samsung Internet?Opera Android12+
beforeunload
onhashchange

Window/hashchange_event

Support in all current engines.

Firefox3.6+Safari5+Chrome8+
Opera10.6+Edge79+
Edge (Legacy)12+Internet Explorer8+
Firefox Android?Safari iOS5+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+
hashchange
onlanguagechange

Window/languagechange_event

Support in all current engines.

Firefox32+Safari10.1+Chrome37+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android4+Safari iOS?Chrome Android?WebView Android?Samsung Internet4.0+Opera Android?
languagechange
onmessage

Window/message_event

Support in all current engines.

Firefox9+Safari4+Chrome60+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer8+
Firefox Android?Safari iOS4+Chrome Android?WebView Android?Samsung Internet?Opera Android47+
message
onmessageerror

HTMLMediaElement/error_event

Support in all current engines.

Firefox6+Safari3.1+Chrome3+
Opera11.6+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android?Samsung Internet?Opera Android12+

Window/messageerror_event

Support in all current engines.

Firefox57+Safari16.4+Chrome60+
Opera?Edge79+
Edge (Legacy)18Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android47+
messageerror
onoffline

Window/offline_event

Support in all current engines.

Firefox9+Safari4+Chrome3+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android?
offline
ononline

Window/online_event

Support in all current engines.

Firefox9+Safari4+Chrome3+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android?
online
onpageswap pageswap
onpagehide pagehide
onpagereveal pagereveal
onpageshow pageshow
onpopstate

Window/popstate_event

Support in all current engines.

Firefox4+Safari5+Chrome5+
Opera11.5+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android11.5+
popstate
onrejectionhandled

Window/rejectionhandled_event

Support in all current engines.

Firefox69+Safari11+Chrome49+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS11.3+Chrome Android?WebView Android?Samsung Internet?Opera Android?
rejectionhandled
onstorage

Window/storage_event

Support in all current engines.

Firefox45+Safari4+Chrome1+
Opera?Edge79+
Edge (Legacy)15+Internet Explorer9+
Firefox Android?Safari iOS4+Chrome Android?WebView Android37+Samsung Internet?Opera Android?
storage
onunhandledrejection

Window/unhandledrejection_event

Support in all current engines.

Firefox69+Safari11+Chrome49+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS11.3+Chrome Android?WebView Android?Samsung Internet?Opera Android?
unhandledrejection
onunload

Window/unload_event

Support in all current engines.

Firefox1+Safari3+Chrome1+
Opera4+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS1+Chrome Android?WebView Android?Samsung Internet?Opera Android10.1+
unload

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

Event handler Event handler event type
onreadystatechange readystatechange
onvisibilitychange

Document/visibilitychange_event

Support in all current engines.

Firefox56+Safari14.1+Chrome62+
Opera49+Edge79+
Edge (Legacy)18Internet Explorer🔰 10+
Firefox Android?Safari iOS?Chrome Android?WebView Android62+Samsung Internet?Opera Android46+
visibilitychange

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 mixin WindowOrWorkerGlobalScope { … }; along with an appropriate reference.

self.isSecureContext

Returns whether or not this global object represents a secure context. [SECURE-CONTEXTS]

self.origin

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

self.crossOriginIsolated

Returns whether scripts running in this global are allowed to use APIs that require cross-origin isolation. This depends on the `Cross-Origin-Opener-Policy` and `Cross-Origin-Embedder-Policy` HTTP response headers and the "cross-origin-isolated" feature.

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.

8.3 Base64 utility methods

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.