1. 8.6 Timers
    2. 8.7 Microtask queuing
    3. 8.8 User prompts
      1. 8.8.1 Simple dialogs
      2. 8.8.2 Printing

8.6 Timers

The setTimeout() and setInterval() methods allow authors to schedule timer-based callbacks.

id = self.setTimeout(handler [, timeout [, ...arguments ] ])

setTimeout

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera4+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android4+Safari iOS1+Chrome Android18+WebView Android1+Samsung Internet1.0+Opera Android10.1+

Schedules a timeout to run handler after timeout milliseconds. Any arguments are passed straight through to the handler.

id = self.setTimeout(code [, timeout ])

Schedules a timeout to compile and run code after timeout milliseconds.

self.clearTimeout(id)

clearTimeout

Support in all current engines.

Firefox1+Safari4+Chrome1+
Opera4+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android4+Safari iOS1+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android10.1+

Cancels the timeout set with setTimeout() or setInterval() identified by id.

id = self.setInterval(handler [, timeout [, ...arguments ] ])

setInterval

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera4+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android4+Safari iOS1+Chrome Android18+WebView Android1+Samsung Internet1.0+Opera Android10.1+

Schedules a timeout to run handler every timeout milliseconds. Any arguments are passed straight through to the handler.

id = self.setInterval(code [, timeout ])

Schedules a timeout to compile and run code every timeout milliseconds.

self.clearInterval(id)

clearInterval

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera4+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android4+Safari iOS1+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android10.1+

Cancels the timeout set with setInterval() or setTimeout() identified by id.

Timers can be nested; after five such nested timers, however, the interval is forced to be at least four milliseconds.

This API does not guarantee that timers will run exactly on schedule. Delays due to CPU load, other tasks, etc, are to be expected.

Objects that implement the WindowOrWorkerGlobalScope mixin have a map of active timers, which is a map, initially empty. Each key in this map is an identifier for a timer, and each value is a DOMHighResTimeStamp, representing the expiry time for that timer.

For entries put in the map of active timers by the timer initialization steps, i.e., by setTimeout() and setInterval(), the keys are numbers. For other specifications that use the run steps after a timeout algorithm, the identifier is a unique non-numeric value. Only the numeric-keyed timers are affected by clearTimeout() and clearInterval(), but all timers contribute to idle deadline computation, and are cleared when the relevant global is destroyed.


The setTimeout(handler, timeout, ...arguments) method steps are to return the result of running the timer initialization steps given this, handler, timeout, arguments, and false.

The setInterval(handler, timeout, ...arguments) method steps are to return the result of running the timer initialization steps given this, handler, timeout, arguments, and true.

The clearTimeout(id) and clearInterval(id) method steps are to remove this's map of active timers[id].

Because clearTimeout() and clearInterval() clear entries from the same map, either method can be used to clear timers created by setTimeout() or setInterval().


The timer initialization steps, given a WindowOrWorkerGlobalScope global, a string or Function handler, a number timeout, a list arguments, a boolean repeat, and optionally (and only if repeat is true) a number previousId, are:

  1. Let thisArg be global if that is a WorkerGlobalScope object; otherwise let thisArg be the WindowProxy that corresponds to global.

  2. If previousId was given, let id be previousId; otherwise, let id be an implementation-defined integer that is greater than zero and does not already exist in global's map of active timers.

  3. If the surrounding agent's event loop's currently running task is a task that was created by this algorithm, then let nesting level be the task's timer nesting level. Otherwise, let nesting level be zero.

    The task's timer nesting level is used both for nested calls to setTimeout(), and for the repeating timers created by setInterval(). (Or, indeed, for any combination of the two.) In other words, it represents nested invocations of this algorithm, not of a particular method.

  4. If timeout is less than 0, then set timeout to 0.

  5. If nesting level is greater than 5, and timeout is less than 4, then set timeout to 4.

  6. Let callerRealm be the current Realm Record, and calleeRealm be global's relevant Realm.

  7. Let initiating script be the active script.

  8. Assert: initiating script is not null, since this algorithm is always called from some script.

  9. Let task be a task that runs the following substeps:

    1. If id does not exist in global's map of active timers, then abort these steps.

    2. If handler is a Function, then invoke handler given arguments with the callback this value set to thisArg. If this throws an exception, catch it, and report the exception.

    3. Otherwise:

      1. Assert: handler is a string.

      2. Perform HostEnsureCanCompileStrings(callerRealm, calleeRealm). If this throws an exception, catch it, report the exception, and abort these steps.

      3. Let settings object be global's relevant settings object.

      4. Let base URL be initiating script's base URL.

      5. Assert: base URL is not null, as initiating script is a classic script or a JavaScript module script.

      6. Let fetch options be a script fetch options whose cryptographic nonce is initiating script's fetch options's cryptographic nonce, integrity metadata is the empty string, parser metadata is "not-parser-inserted", credentials mode is initiating script's fetch options's credentials mode, and referrer policy is initiating script's fetch options's referrer policy.

        The effect of these options ensures that the string compilation done by setTimeout() and setInterval() behaves equivalently to that done by eval(). That is, module script fetches via import() will behave the same in both contexts.

      7. Let script be the result of creating a classic script given handler, settings object, base URL, and fetch options.

      8. Run the classic script script.

    4. If id does not exist in global's map of active timers, then abort these steps.

      It might have been removed via the author code in handler calling clearTimeout() or clearInterval().

    5. If repeat is true, then perform the timer initialization steps again, given global, handler, timeout, arguments, true, and id.

  10. Increment nesting level by one.

  11. Set task's timer nesting level to nesting level.

  12. Let completionStep be an algorithm step which queues a global task on the timer task source given global to run task.

  13. Run steps after a timeout given global, "setTimeout/setInterval", timeout, completionStep, and id.

    Once the task has been processed, if repeat is false, it is safe to remove the entry for id from the map of active timers (there is no way for the entry's existence to be detected past this point, so it does not technically matter one way or the other).

  14. Return id.

Argument conversion as defined by Web IDL (for example, invoking toString() methods on objects passed as the first argument) happens in the algorithms defined in Web IDL, before this algorithm is invoked.

So for example, the following rather silly code will result in the log containing "ONE TWO ":

var log = '';
function logger(s) { log += s + ' '; }

setTimeout({ toString: function () {
  setTimeout("logger('ONE')", 100);
  return "logger('TWO')";
} }, 100);

To run tasks of several milliseconds back to back without any delay, while still yielding back to the browser to avoid starving the user interface (and to avoid the browser killing the script for hogging the CPU), simply queue the next timer before performing work:

function doExpensiveWork() {
  var done = false;
  // ...
  // this part of the function takes up to five milliseconds
  // set done to true if we're done
  // ...
  return done;
}

function rescheduleWork() {
  var id = setTimeout(rescheduleWork, 0); // preschedule next iteration
  if (doExpensiveWork())
    clearTimeout(id); // clear the timeout if we don't need it
}

function scheduleWork() {
  setTimeout(rescheduleWork, 0);
}

scheduleWork(); // queues a task to do lots of work

To run steps after a timeout, given a WindowOrWorkerGlobalScope global, a string orderingIdentifier, a number milliseconds, a set of steps completionSteps, and an optional value timerKey:

  1. Assert: if timerKey is given, then the caller of this algorithm is the timer initialization steps. (Other specifications must not pass timerKey.)

  2. If timerKey is not given, then set it to a new unique non-numeric value.

  3. Let startTime be the current high resolution time.

  4. Set global's map of active timers[timerKey] to startTime plus milliseconds.

  5. Run the following steps in parallel:

    1. If global is a Window object, wait until global's associated Document has been fully active for a further milliseconds milliseconds (not necessarily consecutively).

      Otherwise, global is a WorkerGlobalScope object; wait until milliseconds milliseconds have passed with the worker not suspended (not necessarily consecutively).

    2. Wait until any invocations of this algorithm that had the same global and orderingIdentifier, that started before this one, and whose milliseconds is equal to or less than this one's, have completed.

    3. Optionally, wait a further implementation-defined length of time.

      This is intended to allow user agents to pad timeouts as needed to optimize the power usage of the device. For example, some processors have a low-power mode where the granularity of timers is reduced; on such platforms, user agents can slow timers down to fit this schedule instead of requiring the processor to use the more accurate mode with its associated higher power usage.

    4. Perform completionSteps.

Run steps after a timeout is meant to be used by other specifications that want to execute developer-supplied code after a developer-supplied timeout, in a similar manner to setTimeout(). (Note, however, it does not have the nesting and clamping behavior of setTimeout().) Such specifications can choose an orderingIdentifier to ensure ordering within their specification's timeouts, while not constraining ordering with respect to other specification's timeouts.

8.7 Microtask queuing

queueMicrotask

Support in all current engines.

Firefox69+Safari12.1+Chrome71+
Opera58+Edge79+
Edge (Legacy)NoInternet ExplorerNo
Firefox Android79+Safari iOS12.2+Chrome Android71+WebView Android71+Samsung Internet10.0+Opera Android50+
self.queueMicrotask(callback)

Queues a microtask to run the given callback.

The queueMicrotask(callback) method must queue a microtask to invoke callback, and if callback throws an exception, report the exception.

The queueMicrotask() method allows authors to schedule a callback on the microtask queue. This allows their code to run once the JavaScript execution context stack is next empty, which happens once all currently executing synchronous JavaScript has run to completion. This doesn't yield control back to the event loop, as would be the case when using, for example, setTimeout(f, 0).

Authors ought to be aware that scheduling a lot of microtasks has the same performance downsides as running a lot of synchronous code. Both will prevent the browser from doing its own work, such as rendering. In many cases, requestAnimationFrame() or requestIdleCallback() is a better choice. In particular, if the goal is to run code before the next rendering cycle, that is the purpose of requestAnimationFrame().

As can be seen from the following examples, the best way of thinking about queueMicrotask() is as a mechanism for rearranging synchronous code, effectively placing the queued code immediately after the currently executing synchronous JavaScript has run to completion.

The most common reason for using queueMicrotask() is to create consistent ordering, even in the cases where information is available synchronously, without introducing undue delay.

For example, consider a custom element firing a load event, that also maintains an internal cache of previously-loaded data. A naïve implementation might look like:

MyElement.prototype.loadData = function (url) {
  if (this._cache[url]) {
    this._setData(this._cache[url]);
    this.dispatchEvent(new Event("load"));
  } else {
    fetch(url).then(res => res.arrayBuffer()).then(data => {
      this._cache[url] = data;
      this._setData(data);
      this.dispatchEvent(new Event("load"));
    });
  }
};

This naïve implementation is problematic, however, in that it causes its users to experience inconsistent behavior. For example, code such as

element.addEventListener("load", () => console.log("loaded"));
console.log("1");
element.loadData();
console.log("2");

will sometimes log "1, 2, loaded" (if the data needs to be fetched), and sometimes log "1, loaded, 2" (if the data is already cached). Similarly, after the call to loadData(), it will be inconsistent whether or not the data is set on the element.

To get a consistent ordering, queueMicrotask() can be used:

MyElement.prototype.loadData = function (url) {
  if (this._cache[url]) {
    queueMicrotask(() => {
      this._setData(this._cache[url]);
      this.dispatchEvent(new Event("load"));
    });
  } else {
    fetch(url).then(res => res.arrayBuffer()).then(data => {
      this._cache[url] = data;
      this._setData(data);
      this.dispatchEvent(new Event("load"));
    });
  }
};

By essentially rearranging the queued code to be after the JavaScript execution context stack empties, this ensures a consistent ordering and update of the element's state.

Another interesting use of queueMicrotask() is to allow uncoordinated "batching" of work by multiple callers. For example, consider a library function that wants to send data somewhere as soon as possible, but doesn't want to make multiple network requests if doing so is easily avoidable. One way to balance this would be like so:

const queuedToSend = [];

function sendData(data) {
  queuedToSend.push(data);

  if (queuedToSend.length === 1) {
    queueMicrotask(() => {
      const stringToSend = JSON.stringify(queuedToSend);
      queuedToSend.length = 0;

      fetch("/endpoint", stringToSend);
    });
  }
}

With this architecture, multiple subsequent calls to sendData() within the currently executing synchronous JavaScript will be batched together into one fetch() call, but with no intervening event loop tasks preempting the fetch (as would have happened with similar code that instead used setTimeout()).

8.8 User prompts

8.8.1 Simple dialogs

window.alert(message)

Window/alert

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera3+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android4+Safari iOS1+Chrome Android18+WebView Android1+Samsung Internet1.0+Opera Android10.1+

Displays a modal alert with the given message, and waits for the user to dismiss it.

result = window.confirm(message)

Window/confirm

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera3+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android4+Safari iOS1+Chrome Android18+WebView Android1+Samsung Internet1.0+Opera Android10.1+

Displays a modal OK/Cancel prompt with the given message, waits for the user to dismiss it, and returns true if the user clicks OK and false if the user clicks Cancel.

result = window.prompt(message [, default])

Window/prompt

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera3+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android4+Safari iOS1+Chrome Android18+WebView Android1+Samsung Internet1.0+Opera Android10.1+

Displays a modal text control prompt with the given message, waits for the user to dismiss it, and returns the value that the user entered. If the user cancels the prompt, then returns null instead. If the second argument is present, then the given value is used as a default.

Logic that depends on tasks or microtasks, such as media elements loading their media data, are stalled when these methods are invoked.

The alert() and alert(message) method steps are:

  1. If we cannot show simple dialogs for this, then return.

  2. If the method was invoked with no arguments, then let message be the empty string; otherwise, let message be the method's first argument.

  3. Set message to the result of normalizing newlines given message.

  4. Set message to the result of optionally truncating message.

  5. Show message to the user, treating U+000A LF as a line break.

  6. Optionally, pause while waiting for the user to acknowledge the message.

This method is defined using two overloads, instead of using an optional argument, for historical reasons. The practical impact of this is that alert(undefined) is treated as alert("undefined"), but alert() is treated as alert("").

The confirm(message) method steps are:

  1. If we cannot show simple dialogs for this, then return false.

  2. Set message to the result of normalizing newlines given message.

  3. Set message to the result of optionally truncating message.

  4. Show message to the user, treating U+000A LF as a line break, and ask the user to respond with a positive or negative response.

  5. Pause until the user responds either positively or negatively.

  6. If the user responded positively, return true; otherwise, the user responded negatively: return false.

The prompt(message, default) method steps are:

  1. If we cannot show simple dialogs for this, then return null.

  2. Set message to the result of normalizing newlines given message.

  3. Set message to the result of optionally truncating message.

  4. Set default to the result of optionally truncating default.

  5. Show message to the user, treating U+000A LF as a line break, and ask the user to either respond with a string value or abort. The response must be defaulted to the value given by default.

  6. Pause while waiting for the user's response.

  7. If the user aborts, then return null; otherwise, return the string that the user responded with.

To optionally truncate a simple dialog string s, return either s itself or some string derived from s that is shorter. User agents should not provide UI for displaying the elided portion of s, as this makes it too easy for abusers to create dialogs of the form "Important security alert! Click 'Show More' for full details!".

For example, a user agent might want to only display the first 100 characters of a message. Or, a user agent might replace the middle of the string with "…". These types of modifications can be useful in limiting the abuse potential of unnaturally large, trustworthy-looking system dialogs.

We cannot show simple dialogs for a Window window when the following algorithm returns true:

  1. If the active sandboxing flag set of window's associated Document has the sandboxed modals flag set, then return true.

  2. If window's relevant settings object's origin and window's relevant settings object's top-level origin are not same origin-domain, then return true.

  3. If window's relevant agent's event loop's termination nesting level is nonzero, then optionally return true.
  4. Optionally, return true. (For example, the user agent might give the user the option to ignore all modal dialogs, and would thus abort at this step whenever the method was invoked.)

  5. Return false.

8.8.2 Printing

Window/print

Support in all current engines.

Firefox1+Safari1.1+Chrome1+
Opera6+Edge79+
Edge (Legacy)12+Internet Explorer5+
Firefox AndroidNoSafari iOS1+Chrome Android18+WebView Android1+Samsung Internet1.0+Opera Android10.1+
window.print()

Prompts the user to print the page.

The print() method steps are:

  1. Let document be this's associated Document.

  2. If document is not fully active, then return.

  3. If document's unload counter is greater than 0, then return.

  4. If document is ready for post-load tasks, then run the printing steps for document.

  5. Otherwise, set document's print when loaded flag.

User agents should also run the printing steps whenever the user asks for the opportunity to obtain a physical form (e.g. printed copy), or the representation of a physical form (e.g. PDF copy), of a document.

The printing steps for a Document document are:

  1. The user agent may display a message to the user or return (or both).

    For instance, a kiosk browser could silently ignore any invocations of the print() method.

    For instance, a browser on a mobile device could detect that there are no printers in the vicinity and display a message saying so before continuing to offer a "save to PDF" option.

  2. If the active sandboxing flag set of document has the sandboxed modals flag set, then return.

    If the printing dialog is blocked by a Document's sandbox, then neither the beforeprint nor afterprint events will be fired.

  3. The user agent must fire an event named beforeprint at the relevant global object of document, as well as any child browsing contexts in it.

    The beforeprint event can be used to annotate the printed copy, for instance adding the time at which the document was printed.

  4. The user agent should offer the user the opportunity to obtain a physical form (or the representation of a physical form) of document. The user agent may wait for the user to either accept or decline before returning; if so, the user agent must pause while the method is waiting. Even if the user agent doesn't wait at this point, the user agent must use the state of the relevant documents as they are at this point in the algorithm if and when it eventually creates the alternate form.

  5. The user agent must fire an event named afterprint at the relevant global object of document, as well as any child browsing contexts in it.

    The afterprint event can be used to revert annotations added in the earlier event, as well as showing post-printing UI. For instance, if a page is walking the user through the steps of applying for a home loan, the script could automatically advance to the next step after having printed a form or other.