WindowOrWorkerGlobalScope.setTimeout() - Web APIs 编辑

The setTimeout() method of the WindowOrWorkerGlobalScope mixin (and successor to Window.setTimeout()) sets a timer which executes a function or specified piece of code once the timer expires.

Syntax

var timeoutID = scope.setTimeout(function[, delay, arg1, arg2, ...]);
var timeoutID = scope.setTimeout(function[, delay]);
var timeoutID = scope.setTimeout(code[, delay]);

Parameters

function

A function to be executed after the timer expires.

code

An alternative syntax that allows you to include a string instead of a function, which is compiled and executed when the timer expires. This syntax is not recommended for the same reasons that make using eval() a security risk.

delay Optional

The time, in milliseconds (thousandths of a second), the timer should wait before the specified function or code is executed. If this parameter is omitted, a value of 0 is used, meaning execute "immediately", or more accurately, the next event cycle. Note that in either case, the actual delay may be longer than intended; see Reasons for delays longer than specified below.

arg1, ..., argN Optional

Additional arguments which are passed through to the function specified by function.

Note: Passing additional parameters to the function in the first syntax does not work in Internet Explorer 9 and below. If you want to enable this functionality on that browser, you must use a polyfill (see the Polyfill section).

Return value

The returned timeoutID is a positive integer value which identifies the timer created by the call to setTimeout(); this value can be passed to clearTimeout() to cancel the timeout.

It may be helpful to be aware that setTimeout() and setInterval() share the same pool of IDs, and that clearTimeout() and clearInterval() can technically be used interchangeably. For clarity, however, you should try to always match them to avoid confusion when maintaining your code.

It is guaranteed that a timeout ID will never be reused by a subsequent call to setTimeout() or setInterval() on the same object (a window or a worker). However, different objects use separate pools of IDs.

Example

The following example sets up two simple buttons in a web page and hooks them to the setTimeout() and clearTimeout() routines. Pressing the first button will set a timeout which calls an alert dialog after two seconds and stores the timeout id for use by clearTimeout(). You may optionally cancel this timeout by pressing on the second button.

HTML

<p>Live Example</p>
<button onclick="delayedAlert();">Show an alert box after two seconds</button>
<p></p>
<button onclick="clearAlert();">Cancel alert before it happens</button>

JavaScript

var timeoutID;

function delayedAlert() {
  timeoutID = window.setTimeout(window.alert, 2*1000, 'That was really slow!');
}

function clearAlert() {
  window.clearTimeout(timeoutID);
}

Result

See also clearTimeout() example.

Polyfill

If you need to pass one or more arguments to your callback function, but need it to work in browsers which don't support sending additional arguments using either setTimeout() or setInterval() (e.g., Internet Explorer 9 and below), you can include this polyfill to enable the HTML5 standard arguments-passing functionality. Just add this code to the top of your script:

/*\
|*|
|*|  Polyfill which enables the passage of arbitrary arguments to the
|*|  callback functions of JavaScript timers (HTML5 standard syntax).
|*|
|*|  /wiki/en-US/docs/DOM/window.setInterval
|*|
|*|  Syntax:
|*|  var timeoutID = window.setTimeout(func, delay[, arg1, arg2, ...]);
|*|  var timeoutID = window.setTimeout(code, delay);
|*|  var intervalID = window.setInterval(func, delay[, arg1, arg2, ...]);
|*|  var intervalID = window.setInterval(code, delay);
|*|
\*/

(function() {
  setTimeout(function(arg1) {
    if (arg1 === 'test') {
      // feature test is passed, no need for polyfill
      return;
    }
    var __nativeST__ = window.setTimeout;
    window.setTimeout = function(vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */ ) {
      var aArgs = Array.prototype.slice.call(arguments, 2);
      return __nativeST__(vCallback instanceof Function ? function() {
        vCallback.apply(null, aArgs);
      } : vCallback, nDelay);
    };
  }, 0, 'test');

  var interval = setInterval(function(arg1) {
    clearInterval(interval);
    if (arg1 === 'test') {
      // feature test is passed, no need for polyfill
      return;
    }
    var __nativeSI__ = window.setInterval;
    window.setInterval = function(vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */ ) {
      var aArgs = Array.prototype.slice.call(arguments, 2);
      return __nativeSI__(vCallback instanceof Function ? function() {
        vCallback.apply(null, aArgs);
      } : vCallback, nDelay);
    };
  }, 0, 'test');
}())

IE-only fix

If you want a completely unobtrusive fix for every other mobile or desktop browser, including IE 9 and below, you can either use JavaScript conditional comments:

/*@cc_on
  // conditional IE < 9 only fix
  @if (@_jscript_version <= 9)
  (function(f){
     window.setTimeout = f(window.setTimeout);
     window.setInterval = f(window.setInterval);
  })(function(f){return function(c,t){var a=[].slice.call(arguments,2);return f(function(){c instanceof Function?c.apply(this,a):eval(c)},t)}});
  @end
@*/

... or go for a very clean approach based on the IE HTML conditional feature:

<!--[if lte IE 9]><script>
(function(f){
window.setTimeout=f(window.setTimeout);
window.setInterval=f(window.setInterval);
})(function(f){return function(c,t){
var a=[].slice.call(arguments,2);return f(function(){c instanceof Function?c.apply(this,a):eval(c)},t)}
});
</script><![endif]-->

Workarounds

Another possibility is to use an anonymous function to call your callback, but this solution is a bit more expensive. Example:

var intervalID = setTimeout(function() { myFunc('one', 'two', 'three'); }, 1000);

The above example can also be written with the help of an arrow function:

var intervalID = setTimeout(() => { myFunc('one', 'two', 'three'); }, 1000);

Yet another possibility is to use function's bind. Example:

setTimeout(function(arg1){}.bind(undefined, 10), 1000);

The "this" problem

When you pass a method to setTimeout() (or any other function, for that matter), it will be invoked with a this value that may differ from your expectation. This issue is explained in detail in the JavaScript reference.

Explanation

Code executed by setTimeout() is called from an execution context separate from the function from which setTimeout was called. The usual rules for setting the this keyword for the called function apply, and if you have not set this in the call or with bind, it will default to the window (or global) object. It will not be the same as the this value for the function that called setTimeout.

See the following example:

myArray = ['zero', 'one', 'two'];
myArray.myMethod = function (sProperty) {
  alert(arguments.length > 0 ? this[sProperty] : this);
};

myArray.myMethod(); // prints "zero,one,two"
myArray.myMethod(1); // prints "one"

The above works because when myMethod is called, its this is set to myArray by the call, so within the function, this[sProperty] is equivalent to myArray[sProperty]. However, in the following:

setTimeout(myArray.myMethod, 1.0*1000); // prints "[object Window]" after 1 second
setTimeout(myArray.myMethod, 1.5*1000, '1'); // prints "undefined" after 1.5 seconds

The myArray.myMethod function is passed to setTimeout, then when it's called, its this is not set so it defaults to the window object. There's also no option to pass a thisArg to setTimeout as there is in Array methods like forEach, reduce, etc. and as shown below, using call to set this doesn't work either.

setTimeout.call(myArray, myArray.myMethod, 2.0*1000); // error: "NS_ERROR_XPC_BAD_OP_ON_WN_PROTO: Illegal operation on WrappedNative prototype object"
setTimeout.call(myArray, myArray.myMethod, 2.5*1000, 2); // same error

Possible solutions

A common way to solve the problem is to use a wrapper function that sets this to the required value:

setTimeout(function(){myArray.myMethod()}, 2.0*1000); // prints "zero,one,two" after 2 seconds
setTimeout(function(){myArray.myMethod('1')}, 2.5*1000); // prints "one" after 2.5 seconds

Arrow functions are a possible alternative, too:

setTimeout(() => {myArray.myMethod()}, 2.0*1000); // prints "zero,one,two" after 2 seconds
setTimeout(() => {myArray.myMethod('1')}, 2.5*1000); // prints "one" after 2.5 seconds

Another possible way to solve the "this" problem is to replace the host setTimeout() and setInterval() global functions with ones that allow passing a this object and set it in the callback using Function.prototype.call, e.g.:

// Enable setting 'this' in JavaScript timers

var __nativeST__ = window.setTimeout,
    __nativeSI__ = window.setInterval;

window.setTimeout = function (vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */) {
  var oThis = this,
      aArgs = Array.prototype.slice.call(arguments, 2);
  return __nativeST__(vCallback instanceof Function ? function () {
    vCallback.apply(oThis, aArgs);
  } : vCallback, nDelay);
};

window.setInterval = function (vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */) {
  var oThis = this,
      aArgs = Array.prototype.slice.call(arguments, 2);
  return __nativeSI__(vCallback instanceof Function ? function () {
    vCallback.apply(oThis, aArgs);
  } : vCallback, nDelay);
};

Note: These two replacements will also enable the HTML5 standard passage of arbitrary arguments to the callback functions of timers in IE. So they can be used as polyfills also. See the Callback arguments paragraph.

New feature test:

myArray = ['zero', 'one', 'two'];
myArray.myMethod = function (sProperty) {
    alert(arguments.length > 0 ? this[sProperty] : this);
};

setTimeout(alert, 1500, 'Hello world!'); // the standard use of setTimeout and setInterval is preserved, but...
setTimeout.call(myArray, myArray.myMethod, 2.0*1000); // prints "zero,one,two" after 2 seconds
setTimeout.call(myArray, myArray.myMethod, 2.5*1000, 2); // prints "two" after 2.5 seconds

Note: JavaScript 1.8.5 introduced the Function.prototype.bind() method to set the value of this for all calls to a given function. This can avoid having to use a wrapper function to set the value of this in a callback.

Example using bind():

myArray = ['zero', 'one', 'two'];
myBoundMethod = (function (sProperty) {
    console.log(arguments.length > 0 ? this[sProperty] : this);
}).bind(myArray);

myBoundMethod(); // prints "zero,one,two" because 'this' is bound to myArray in the function
myBoundMethod(1); // prints "one"
setTimeout(myBoundMethod, 1.0*1000); // still prints "zero,one,two" after 1 second because of the binding
setTimeout(myBoundMethod, 1.5*1000, "1"); // prints "one" after 1.5 seconds

Notes

Timeouts are cancelled using clearTimeout().

To call a function repeatedly (e.g., every N milliseconds), consider using setInterval().

Passing string literals

Passing a string instead of a function to setTimeout() has the same associated problems as using eval.

// Recommended
window.setTimeout(function() {
  alert('Hello World!');
}, 500);

// Not recommended
window.setTimeout("alert('Hello World!');", 500);

A string passed to setTimeout is evaluated in the global context, so local symbols in the context where setTimeout() was called will not be available when the string is evaluated as code.

Reasons for delays longer than specified

There are a number of reasons why a timeout may take longer to fire than anticipated. This section describes the most common reasons.

Timeouts throttled to ≥ 4ms

In modern browsers, setTimeout()/setInterval() calls are throttled to a minimum of once every 4 ms when successive calls are triggered due to callback nesting (where the nesting level is at least a certain depth), or after certain number of successive intervals. Take this example:

function cb() { setTimeout(cb, 0); }
function f() { setInterval(f, 0); }
setTimeout(cb, 0);
setInterval(f, 0);

In Chrome and Firefox, the 5th successive callback call is clamped; Safari clamps on the 6th call; in Edge its the 3rd one. Gecko started to treat setInterval() like this in version 56 (it already did this with setTimeout(); see below).

Historically some browsers implemented this throttling behaviour a bit differently (e.g. Firefox) — on setInterval() calls made from anywhere, or when a nested setTimeout() is called where the nesting level is at least a certain depth.

To implement a 0 ms timeout in a modern browser, you can use window.postMessage() as described here.

Note: The minimum delay, DOM_MIN_TIMEOUT_VALUE, is 4 ms (stored in a preference in Firefox: dom.min_timeout_value), with a DOM_CLAMP_TIMEOUT_NESTING_LEVEL of 5.

Note: 4 ms is specified by the HTML5 spec and is consistent across browsers released in 2010 and onward. Prior to (Firefox 5.0 / Thunderbird 5.0 / SeaMonkey 2.2), the minimum timeout value for nested timeouts was 10 ms.

Timeouts in inactive tabs throttled to ≥ 1000ms

To reduce the load (and associated battery usage) from background tabs, timeouts are throttled to firing no more often than once per second (1,000 ms) in inactive tabs.

Firefox implements this behavior since version 5 (see bug 633421, the 1000ms constant can be tweaked through the dom.min_background_timeout_value preference). Chrome implements this behavior since version 11 (crbug.com/66078).

Firefox for Android uses a timeout value of 15 minutes for background tabs since bug 736602 in Firefox 14, and background tabs can also be unloaded entirely.

Firefox 50 no longer throttles background tabs if a Web Audio API AudioContext is actively playing sound. Firefox 51 further amends this such that background tabs are no longer throttled if an AudioContext is present in the tab at all, even if no sound is being played. These resolve a number of issues with apps which play note-based music not being able to time or synchronize the music properly when the tab is in the background.

Throttling of tracking timeout scripts

Since Firefox 55, tracking scripts (e.g. Google Analytics, any script URL that Firefox recognises as a tracking script through its TP lists) have been subject to further throttling. When running in the foreground, the throttling minimum delay is still 4ms. In background tabs, however, the throttling minimum delay is 10,000 ms, or 10 seconds, which comes into effect 30 seconds after a document has first loaded.

The prefs that control this behaviour are:

• dom.min_tracking_timeout_value: 4
• dom.min_tracking_background_timeout_value: 10000
• dom.timeout.tracking_throttling_delay: 30000

Late timeouts

In addition to "clamping", the timeout can also fire later when the page (or the OS/browser itself) is busy with other tasks. One important case to note is that the function or code snippet cannot be executed until the thread that called setTimeout() has terminated. For example:

function foo() {
  console.log('foo has been called');
}
setTimeout(foo, 0);
console.log('After setTimeout');

Will write to the console:

After setTimeout
foo has been called

This is because even though setTimeout was called with a delay of zero, it's placed on a queue and scheduled to run at the next opportunity; not immediately. Currently-executing code must complete before functions on the queue are executed, thus the resulting execution order may not be as expected.

Deferral of timeouts during pageload

Starting in Firefox 66, Firefox will defer firing setTimeout and setInterval timers while the current tab is loading.   Firing is deferred until the MainThread is deemed idle (similar to window.requestIdleCallback()), or until the load event is fired.

WebExtension background pages and timers

In WebExtension background pages, timers don't work properly. This is because background pages don't actually stay loaded all the time: the browser can unload them if they are not being used, and restore them when they are needed. This is mostly transparent to the extension, but some things (including JS timers) won't work across an unload/restore cycle, so background pages are encouraged instead to use alarms, which will work. There's some more detail on this at Migrate to Event Driven Background Scripts.

At the time of writing, only Chrome exhibited the above behaviour — Firefox doesn't yet do the unload/restore behavior, so timers will work. But it's still a good idea to avoid timers in WebExtensions, for a couple of reasons:

1. Compat with Chrome.
2. Future changes in behavior that may cause problems.

Maximum delay value

Browsers including Internet Explorer, Chrome, Safari, and Firefox store the delay as a 32-bit signed integer internally. This causes an integer overflow when using delays larger than 2,147,483,647 ms (about 24.8 days), resulting in the timeout being executed immediately.

Specifications

Specification	Status	Comment
HTML Living Standard The definition of 'WindowOrWorkerGlobalScope.setTimeout()' in that specification.	Living Standard	Method moved to the WindowOrWorkerGlobalScope mixin in the latest spec.
HTML Living Standard The definition of 'WindowTimers.setTimeout()' in that specification.	Living Standard	Initial definition (DOM Level 0)

Browser compatibility

BCD tables only load in the browser

See also

• JavaScript timers
• Timer.jsm
• WindowOrWorkerGlobalScope.clearTimeout
• WindowOrWorkerGlobalScope.setInterval
• window.requestAnimationFrame
• Daemons management