Window: setInterval() method
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The setInterval()
method of the Window
interface 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
setInterval(code)
setInterval(code, delay)
setInterval(func)
setInterval(func, delay)
setInterval(func, delay, arg1)
setInterval(func, delay, arg1, arg2)
setInterval(func, delay, arg1, arg2, /* …, */ argN)
Parameters
func
-
A
function
to be executed everydelay
milliseconds. The first execution happens afterdelay
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 usingeval()
a security risk. delay
Optional-
The time, in milliseconds (thousandths of a second), the timer should delay in between executions of the specified function or code. Defaults to 0 if not specified. 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.
Note: The delay
argument is converted to a signed 32-bit integer.
This effectively limits delay
to 2147483647 ms, roughly 24.8 days, since it's specified as a signed integer in the IDL.
Examples
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 nIntervId;
function changeColor() {
// check if an interval has already been set up
if (!nIntervId) {
nIntervId = setInterval(flashText, 1000);
}
}
function flashText() {
const oElem = document.getElementById("my_box");
oElem.className = oElem.className === "go" ? "stop" : "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
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
All modern JavaScript runtimes (in browsers and elsewhere) support arrow functions, with lexical this
— allowing us to write setInterval(() => this.myMethod())
if we're inside the myArray
method.
If you need to support IE, use the Function.prototype.bind()
method, which lets you specify the value that should be used as this
for all calls to a given function. That lets you easily bypass problems where it's unclear what this
will be, depending on the context from which your function was called.
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 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(() => {
// 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
Specification |
---|
HTML Standard # dom-setinterval-dev |
Browser compatibility
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