setInterval()

The setInterval() method, offered on the Window and Worker interfaces, repeatedly calls a function or executes a code snippet, with a fixed time delay between each call.

This method returns an interval ID which uniquely identifies the interval, so you can remove it later by calling clearInterval().

Syntax

var intervalID = setInterval(func, [delay, arg1, arg2, ...]);
var intervalID = setInterval(function[, delay]);
var intervalID = setInterval(code, [delay]);

Parameters

func

A function to be executed every delay milliseconds. The first execution happens after delay milliseconds.

code

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

delayOptional

The time, in milliseconds (thousandths of a second), the timer should delay in between executions of the specified function or code. See Delay restrictions below for details on the permitted range of delay values.

arg1, ..., argN Optional

Additional arguments which are passed through to the function specified by func once the timer expires.

Return value

The returned intervalID is a numeric, non-zero value which identifies the timer created by the call to setInterval(); this value can be passed to clearInterval() to cancel the interval.

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

Examples

Example 1: Basic syntax

The following example demonstrates setInterval()'s basic syntax.

var intervalID = setInterval(myCallback, 500, 'Parameter 1', 'Parameter 2');

function myCallback(a, b)
{
 // Your code here
 // Parameters are purely optional.
 console.log(a);
 console.log(b);
}

Example 2: Alternating two colors

The following example calls the flashtext() function once a second until the Stop button is pressed.

HTML

<div id="my_box">
  <h3>Hello World</h3>
</div>
<button id="start">Start</button>
<button id="stop">Stop</button>

CSS

.go {
  color: green;
}
.stop {
  color: red;
}

JavaScript

// variable to store our intervalID
let nIntervId;

function changeColor() {
  // check if already an interval has been set up
  if (!nIntervId) {
    nIntervId = setInterval(flashText, 1000);
  }
}

function flashText() {
  const oElem = document.getElementById("my_box");
  if (oElem.className === "go") {
    oElem.className = "stop";
  } else {
    oElem.className = "go";
  }
}

function stopTextColor() {
  clearInterval(nIntervId);
  // release our intervalID from the variable
  nIntervId = null; 
}

document.getElementById("start").addEventListener("click", changeColor);
document.getElementById("stop").addEventListener("click", stopTextColor);

Result

See also: clearInterval().

Callback arguments

As previously discussed, Internet Explorer versions 9 and below do not support the passing of arguments to the callback function in either setTimeout() or setInterval(). The following IE-specific code demonstrates a method for overcoming this limitation. To use, add the following code to the top of your script.

/*\
|*|
|*|  IE-specific polyfill that enables the passage of arbitrary arguments to the
|*|  callback functions of javascript timers (HTML5 standard syntax).
|*|
|*|  https://developer.mozilla.org/en-US/docs/Web/API/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);
|*|
\*/

if (document.all && !window.setTimeout.isPolyfill) {
  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);
  };
  window.setTimeout.isPolyfill = true;
}

if (document.all && !window.setInterval.isPolyfill) {
  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);
  };
  window.setInterval.isPolyfill = true;
}

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

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

Another possibility is to use function's bind. Example:

var intervalID = setInterval(function(arg1) {}.bind(undefined, 10), 1000);

Starting in Gecko 5.0 (Firefox 5.0 / Thunderbird 5.0 / SeaMonkey 2.2), intervals are clamped to fire no more often than once per second in inactive tabs.

The "this" problem

When you pass a method to setInterval() or any other function, it is invoked with the wrong this value. This problem is explained in detail in the JavaScript reference.

Explanation

Code executed by setInterval() runs in a separate execution context than the function from which it was called. As a consequence, the this keyword for the called function is set to the window (or global) object, it is not the same as the this value for the function that called setTimeout. See the following example (which uses setTimeout() instead of setInterval() – the problem, in fact, is the same for both timers):

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"
setTimeout(myArray.myMethod, 1000); // prints "[object Window]" after 1 second
setTimeout(myArray.myMethod, 1500, "1"); // prints "undefined" after 1,5 seconds
// passing the 'this' object with .call won't work
// because this will change the value of this inside setTimeout itself
// while we want to change the value of this inside myArray.myMethod
// in fact, it will be an error because setTimeout code expects this to be the window object:
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

As you can see there are no ways to pass the this object to the callback function in the legacy JavaScript.

A possible solution

A possible way to solve the "this" problem is to replace the two native setTimeout() or setInterval() global functions with two non-native ones that enable their invocation through the Function.prototype.call method. The following example shows a possible replacement:

// Enable the passage of the 'this' object through the 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);
};

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

Usage notes

The setInterval() function is commonly used to set a delay for functions that are executed again and again, such as animations. You can cancel the interval using clearInterval().

If you wish to have your function called once after the specified delay, use setTimeout().

Delay restrictions

It's possible for intervals to be nested; that is, the callback for setInterval() can in turn call setInterval() to start another interval running, even though the first one is still going. To mitigate the potential impact this can have on performance, once intervals are nested beyond five levels deep, the browser will automatically enforce a 4 ms minimum value for the interval. Attempts to specify a value less than 4 ms in deeply-nested calls to setInterval() will be pinned to 4 ms.

Browsers may enforce even more stringent minimum values for the interval under some circumstances, although these should not be common. Note also that the actual amount of time that elapses between calls to the callback may be longer than the given delay; see Reasons for delays longer than specified in setTimeout() for examples.

Ensure that execution duration is shorter than interval frequency

If there is a possibility that your logic could take longer to execute than the interval time, it is recommended that you recursively call a named function using setTimeout(). For example, if using setInterval() to poll a remote server every 5 seconds, network latency, an unresponsive server, and a host of other issues could prevent the request from completing in its allotted time. As such, you may find yourself with queued up XHR requests that won't necessarily return in order.

In these cases, a recursive setTimeout() pattern is preferred:

(function loop(){
   setTimeout(function() {
      // Your logic here

      loop();
  }, delay);
})();

In the above snippet, a named function loop() is declared and is immediately executed. loop() is recursively called inside setTimeout() after the logic has completed executing. While this pattern does not guarantee execution on a fixed interval, it does guarantee that the previous interval has completed before recursing.

Specifications

Browser compatibility

See also

• A polyfill of setInterval which allows passing arguments to the callback is available in core-js
• setTimeout
• clearTimeout
• clearInterval
• window.requestAnimationFrame