Window: setInterval() method

Syntax

setInterval(code)
setInterval(code, delay)

setInterval(func)
setInterval(func, delay)
setInterval(func, delay, param1)
setInterval(func, delay, param1, param2)
setInterval(func, delay, param1, param2, /* …, */ paramN)

Parameters

func

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

code

A TrustedScript or a string of arbitrary code that is compiled and executed every delay milliseconds. This can be used instead of passing a function, but is strongly discouraged for the same reasons that make using eval() a security risk.

delay Optional

The delay time between executions of the specified function or code, in milliseconds. Defaults to 0 if not specified. See Delay restrictions below for details on the permitted range of delay values.

param1, …, paramN Optional

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

Return value

A positive integer (typically within the range of 1 to 2,147,483,647) that uniquely identifies the interval timer created by the call.

This identifier, often referred to as an "interval ID", can be passed to clearInterval() to stop the repeated execution of the specified function.

Exceptions

SyntaxError

The code can't be parsed as a script.

TypeError

Thrown if the code parameter is set to a string when Trusted Types are enforced by a CSP and no default policy is defined. It is also thrown if the first parameter is not one of the supported types: a function, string or TrustedScript.

Description

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 for examples.

Interval IDs are shared with setTimeout()

The method returns an identifier that uniquely identifies the interval timer created by the call. This identifier, which is often referred to as an "interval ID", can be passed to clearInterval() to stop the repeated execution of the specified function.

Within the same global environment (e.g., a particular window or worker), the interval ID is ensured to remain unique and is not reused for any new interval timer as long as the original timer is still active. However, different global environments maintain their own independent pools of interval IDs.

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.

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(() => {
    // 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.

Functions are called with the global this

The functions passed to setInterval() is run with normal function call semantics for determining the reference of this. This problem is explained in detail in the JavaScript reference.

For non-arrow functions, the this context is set to the globalThis (an alias for window in browsers) object.

The following example demonstrates how this can cause unexpected behavior. Here, when we pass the method counter.count directly to setInterval(), the this context is lost, and the method is called on the global object instead of the Counter instance, resulting in a TypeError when the count method tries to access this:

class Counter {
  constructor() {
    this.data = new Map();
  }

  count(item) {
    this.data.set(item, (this.data.get(item) || 0) + 1);
  }
}

const counter = new Counter();

counter.count("foo"); // Successfully adds "foo" to the map
setInterval(counter.count, 1000, "bar");
// TypeError: Cannot read properties of undefined (reading 'set')

To work around this, you must make sure that the function passed to setInterval has the correct this context. There are three main ways to do this:

1. If you want to explicitly specify the this context, instead of passing the method directly, wrap the method call in another anonymous function that explicitly calls the method with the correct context:

   js

   setInterval(() => counter.count("bar"), 1000);
   setInterval(function () {
     counter.count("bar");
   }, 1000);
   

2. If you want to use the this context of the code that calls setInterval(), always use an arrow function, which inherits the this context of its enclosing scope:

   js

   class Counter {
     // …
     repeatedCount(item) {
       // BAD: the `this` context is lost in the callback
       setInterval(function () {
         this.data.set(item, (this.data.get(item) || 0) + 1);
       }, 1000);
     }
   }
   

   js

   class Counter {
     // …
     repeatedCount(item) {
       // GOOD: the arrow function inherits the `this` context of `repeatedCount()`
       setInterval(() => {
         this.data.set(item, (this.data.get(item) || 0) + 1);
       }, 1000);
     }
   }
   

3. If you want to avoid extra function wrappers (which increase memory usage) while explicitly specifying the this context, you can use the Function.prototype.bind() method to create a new function with the correct this context:

   js

   setInterval(counter.count.bind(counter), 1000, "bar");
   

Security considerations

The method can be used to execute arbitrary input passed in the code parameter. If the input is a potentially unsafe string provided by a user, this is a possible vector for Cross-site-scripting (XSS) attacks.

For example, the following code shows how setInterval() might execute untrustedCode provided by a user:

const untrustedCode = "alert('Potentially evil code!');";
const id = setInterval(untrustedCode, 1000);

Websites with a Content Security Policy (CSP) that specifies script-src or default-src will prevent such code running by default. You can specify unsafe-eval in your CSP to allow setInterval() to execute, but this is unsafe as it disables one of the main protections of CSP.

See Inline JavaScript in the CSP guide.

If you must allow the scripts to run via setInterval() you can mitigate these issues by always assigning TrustedScript objects instead of strings, and enforcing trusted types using the require-trusted-types-for CSP directive. This ensures that the input is passed through a transformation function.

To allow setInterval() to run, you will additionally need to specify the trusted-types-eval keyword in your CSP script-src directive. This acts in the same way as unsafe-eval, but only allows the method to evaluate if trusted types are enabled (if you were to use unsafe-eval it would allow execution even on browsers that do not support trusted types).

For example, the required CSP for your site might look like this:

Content-Security-Policy: require-trusted-types-for 'script'; script-src '<your_allowlist>' 'trusted-types-eval'

The behavior of the transformation function will depend on the specific use case that requires a user provided script. If possible you should lock the allowed scripts to exactly the code that you trust to run. If that is not possible, you might allow or block the use of certain functions within the provided string.

Examples

Note that these examples omit the use of trusted types for brevity. See Using TrustedScript in eval() for code showing the expected approach.

Example 1: Basic syntax

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

const 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 intervalId;

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

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

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

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

Result

Specifications

Browser compatibility

See also

• Polyfill of setInterval which allows passing arguments to the callback in core-js
• Window.clearInterval()
• WorkerGlobalScope.setInterval()
• Window.setTimeout()
• Window.requestAnimationFrame()