Discover JavaScript Timers

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When writing JavaScript code, you might want to delay the execution of a function.

This is the job of setTimeout. You specify a callback function to execute later, and a value expressing how later you want it to run, in milliseconds:


This syntax defines a new function. You can call whatever other function you want in there, or you can pass an existing function name, and a set of parameters:


setTimeout returns the timer id. This is generally not used, but you can store this id, and clear it if you want to delete this scheduled function execution:


Zero delay

If you specify the timeout delay to 0, the callback function will be executed as soon as possible, but after the current function execution:


This code will print


This is especially useful to avoid blocking the CPU on intensive tasks and let other functions be executed while performing a heavy calculation, by queuing functions in the scheduler.

Some browsers (IE and Edge) implement a setImmediate() method that does this same exact functionality, but it's not standard and unavailable on other browsers. But it's a standard function in Node.js.


setInterval is a function similar to setTimeout, with a difference: instead of running the callback function once, it will run it forever, at the specific time interval you specify (in milliseconds):


The function above runs every 2 seconds unless you tell it to stop, using clearInterval, passing it the interval id that setInterval returned:


It's common to call clearInterval inside the setInterval callback function, to let it auto-determine if it should run again or stop. For example this code runs something unless App.somethingIWait has the value arrived:


Recursive setTimeout

setInterval starts a function every n milliseconds, without any consideration about when a function finished its execution.

If a function always takes the same amount of time, it's all fine:

setInterval working fine

Maybe the function takes different execution times, depending on network conditions for example:

setInterval varying duration

And maybe one long execution overlaps the next one:

setInterval overlapping

To avoid this, you can schedule a recursive setTimeout to be called when the callback function finishes:


to achieve this scenario:

Recursive setTimeout

setTimeout and setInterval are available in Node.js, through the Timers module.

Node.js also provides setImmediate(), which is equivalent to using setTimeout(() => {}, 0), mostly used to work with the Node.js Event Loop.