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Calls a function or executes a code snippet after a specified delay.

Syntax

var timeoutID = window.setTimeout(func, [delay, param1, param2, ...]);
var timeoutID = window.setTimeout(code, [delay]);

where

  • timeoutID is the numerical ID of the timeout, which can be used later with window.clearTimeout().
  • func is the function you want to execute after delay milliseconds.
  • code in the alternate syntax is a string of code you want to execute after delay milliseconds (using this syntax is not recommended for the same reasons as using eval())
  • delay is the number of milliseconds (thousandths of a second) that the function call should be delayed by. If omitted, it defaults to 0. The actual delay may be longer; see Notes below.
  • param1, param2, and so forth are additional parameters which are passed through to the function specified by func.

Note that 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 Callback arguments section).

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 Content

<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 Content

var timeoutID;

function delayedAlert() {
  timeoutID = window.setTimeout(slowAlert, 2000);
}

function slowAlert() {
  alert("That was really slow!");
}

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

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 parameters using either setTimeout() or setInterval() (e.g. Internet Explorer 9 and below), you can include this polyfill which enables the HTML5 standard parameter-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).
|*|
|*|  https://developer.mozilla.org/en-US/docs/DOM/window.setInterval
|*|
|*|  Syntax:
|*|  var timeoutID = window.setTimeout(func, delay, [param1, param2, ...]);
|*|  var timeoutID = window.setTimeout(code, delay);
|*|  var intervalID = window.setInterval(func, delay[, param1, param2, ...]);
|*|  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 hack 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 a separate execution context to 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 global (or window) object in non–strict mode, or be undefined in strict mode. 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, 1000); // prints "[object Window]" after 1 second
setTimeout(myArray.myMethod, 1500, "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, 2000); // error: "NS_ERROR_XPC_BAD_OP_ON_WN_PROTO: Illegal operation on WrappedNative prototype object"
setTimeout.call(myArray, myArray.myMethod, 2500, 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()}, 2000); // prints "zero,one,two" after 2 seconds
setTimeout(function(){myArray.myMethod('1')}, 2500); // prints "one" after 2.5 seconds

Arrow functions are a possible alternative, too:

setTimeout(() => {myArray.myMethod()}, 2000); // prints "zero,one,two" after 2 seconds
setTimeout(() => {myArray.myMethod('1')}, 2500); // 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, 2000); // prints "zero,one,two" after 2 seconds
setTimeout.call(myArray, myArray.myMethod, 2500, 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, 1000); // still prints "zero,one,two" after 1 second because of the binding
setTimeout(myBoundMethod, 1500, "1"); // prints "one" after 1.5 seconds

Notes

Timeouts are cancelled using window.clearTimeout(). To call a function repeatedly (e.g., every N milliseconds), consider using window.setInterval().

It's important to note 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

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, the resulting execution order may not be as expected.

Passing string literals

Passing a string instead of a function to setTimeout() suffers from the same hazards 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

Nested timeouts forced to >=4ms

Historically browsers implement setTimeout() "clamping": successive setTimeout() calls with delay smaller than the "minimum delay" limit are forced to use at least the minimum delay. 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.

In fact, 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.

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

Timeouts in inactive tabs clamped to >=1000ms

To reduce the load (and associated battery usage) from background tabs, timeouts are often clamped to firing no more often than once per second (1000 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.

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.

Prior to Gecko 13 (Firefox 13.0 / Thunderbird 13.0 / SeaMonkey 2.10), Gecko passed an extra parameter to the callback routine, indicating the "actual lateness" of the timeout in milliseconds. This non-standard parameter is no longer passed.

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 2147483647, resulting in the timeout being executed immediately.

Specifications

Specification Status Comment
WHATWG HTML Living Standard
The definition of 'WindowTimers.setTimeout()' in that specification.
Living Standard Initial definition (DOM Level 0)

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 1.0 1.0 (1.7 or earlier) 4.0 4.0 1.0
Supports parameters for callback[1] (Yes) (Yes) 10.0 (Yes) (Yes)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support 1.0 1.0 1.0 (1) 6.0 6.0 1.0
Supports parameters for callback[1] ? ? ? ? ? ?

[1] Whether it supports the optional parameters when in its first form or not.

See also