What does setInterval() do?

JavaScriptJunior~1 min read

What `setInterval()` does

setInterval() is a built-in JavaScript function that:

starts a periodic timer,
every delay milliseconds adds the given callback to the macrotask queue,
keeps doing this until clearInterval() is called.

Syntax

javascript

setInterval(callback, delay, ...args);

Parameters:

callback - the function to call repeatedly;
delay - the interval between runs, in milliseconds;
...args - extra arguments passed to the callback.

Example

javascript

let count = 0;

const id = setInterval(() => {
  count++;
  console.log(`${count} seconds have passed`);

  if (count === 5) {
    clearInterval(id);
    console.log('Timer stopped');
  }
}, 1000);

What happens:

javascript

seconds have passed
seconds have passed
seconds have passed
seconds have passed
seconds have passed
Timer stopped

Every second a new function is added to the task queue, and it runs when the stack is empty.

What `setInterval()` returns

It returns a numeric interval identifier (ID) - the same type as setTimeout() returns.

This ID is used to stop the timer:

javascript

const intervalId = setInterval(() => console.log('tick'), 1000);

// stop it after 5 seconds
setTimeout(() => clearInterval(intervalId), 5000);

How it works under the hood

setInterval() registers the timer with an external API (Web API / Node Timer API).
Every delay ms the Web API adds the callback to the macrotask queue (Task Queue).
The Event Loop runs the callback when the call stack is empty.
The cycle repeats until clearInterval() is called.

Important to know

1. Interval does not mean exact timing

setInterval does not guarantee the callback fires exactly after delay ms. If the engine is busy (for example, running heavy code), the run is delayed.

javascript

setInterval(() => console.log('tick'), 1000);
while (true) {} // the page "hangs", intervals never run

2. Calls can pile up

If the callback takes longer than the interval itself, tasks will pile up in the queue:

javascript

setInterval(() => {
  const start = Date.now();
  while (Date.now() - start < 2000) {} // blocks the thread for 2 sec
  console.log('Call');
}, 1000);

The interval is 1 s, but execution takes 2 s - callbacks will "crowd" the queue and run one after another, with no pauses.

3. You can pass arguments

javascript

setInterval((a, b) => console.log(a + b), 1000, 2, 3);
// -> Every second it prints 5

Difference between `setInterval()` and `setTimeout()`

Property	`setTimeout()`	`setInterval()`
Runs	Once	Repeatedly
Repetition	No	Yes
Returns	Timer ID	Interval ID
Cancellation	`clearTimeout(id)`	`clearInterval(id)`
Queue	Macrotasks	Macrotasks
Used for	Deferred execution	Repeating tasks

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