setTimeout(fn, 0) !== 0ms

When working with timers, setting a delay of 0, does not equate to 0ms or “instant” execution. This longer than expected delay may occur because the OS/browser/system is busy with other tasks or because the timer has been throttled.

Note: for the purpose of this discussion “instantaneous” or “instant” means code that can run immediately (e.g. code that was not placed on a queue by setTimeout or setInterval)

Timeout Throttling

Each JavaScript environment, be it the browser or Node.js, throttles setTimeout and setInterval. This throttle means that setTimeout and setInterval have a minimum delay that is greater than 0ms.

The minimum delay is:

  • 4ms in browsers (per MDN)
    • This throttle occurs when successive calls are triggered due to callback nesting or after a certain number of successive intervals
  • 1ms in Node.js (per Node docs)

The implication being that setting a delay of 0ms will not happen instantaneously.

Late timeouts

From MDN

The timeout can also fire later when the page (or the OS/browser itself) is busy with other tasks. One important case to note is that the function or code snippet cannot be executed until the thread that called setTimeout() has terminated.

For example:

function runAwesomeCode() {
    console.log('Everything is awesome');
setTimeout(runAwesomeCode, 0);
console.log('Awesomeness completed');

Results in

Awesomeness completed
Everything is awesome

Not quite what we expected, is it? This is because even though we set a delay of 0, the code within a timer is placed on a queue and scheduled to run at the next opportunity, not immediately. Code that is currently executing must complete before functions on the queue are run.

Background aka “how this arose during development”

At my company, one of our current strategic initiatives is to enhance the scalability of our infrastructure. To that end, members of the DevOps team are conducting performance tests on our servers to better understand the maximum user load we can support while maintaining our KPIs. To facilitate this testing, the team is building a tool that simulates a play-through on our web application and randomizes when certain actions (e.g. button clicks, data submissions) occur. Once completed, this tool will be “spawned” across multiple server instances to hammer our infrastructure :).

A requirement of this tool is to both run “instantly” (to maximize hits per minute) and “randomly” (within specified ranges of time) to simulate user interaction.

While testing, we observed that when setting a timeout of 0, the overall play-through was taking longer than expected. After further research, we discovered that yes, a delay of 0, does not equal 0ms and is definitely not “instant”.

Now that we have a little background, let’s dive into tests we conducted to illustrate the minimum delay used by setTimeout and setInterval.

Testing details and steps:

System Specifications

  • Computer: Dell Latitude E5570. 16MB RAM, Processor: i7 2.60GHz. Windows 10
  • Chrome - Version 66.0.3359.139
  • Firefox - Version 60.0
  • Node.js - Version 10.1.0

How the tests were performed:

In the browser:
run the following steps 5 times for each file

In Node.js:
run each statement below 5 times

node loop.js
node loop-settimeout.js
node loop-setinterval.js

Test Code


const arr = new Array(10000);
for (let i = 0; i < arr.length; i++) {
  arr[i] = new Object();


const createObject = element => {
  element = new Object();

const arr = new Array(10000);
for (let i = 0; i < arr.length; i++) {
  setTimeout(createObject, 0, arr[i]);


const numElements = 10000;
let count = 0;
let intervalID;

const createObject = element => {
  if (count !== numElements-1) {
    element = new Object();
  else {

const arr = new Array(numElements);
intervalID = setInterval(createObject, 0, arr[count]);

The code can be found here.

Test Results


Note: Chrome provides more significant digits than Firefox and thus the times are rounded to the nearest whole number to provide equivalent levels of precision.

Run loop.js loop-settimeout.js loop-setinterval.js
1 2ms 66ms 40289ms
2 4ms 74ms 40131ms
3 7ms 62ms 40210ms
4 3ms 65ms 40117ms
5 4ms 68ms 40169ms


Run loop.js loop-settimeout.js loop-setinterval.js
1 4ms 26ms 44678ms
2 4ms 21ms 45246ms
3 5ms 21ms 43100ms
4 2ms 23ms 44167ms
5 3ms 22ms 44413ms


Run loop.js loop-settimeout.js loop-setinterval.js
1 0.592ms 18.832ms 27777.110ms
2 0.574ms 17.423ms 27618.277ms
3 0.586ms 16.748ms 25347.014ms
4 0.702ms 18.811ms 27398.416ms
5 0.583ms 16.926ms 25325.018ms


By comparing loops that do and do not utilize timers, our test results confirm that setTimeout and setInterval indeed have a delay that is longer than expected when the delay is set to 0. The setInterval results from the browser tests roughly equal 4ms (i.e. the throttle) multiplied by 10,000 (i.e. the length of the loop). In both Node.js and browser tests, setTimeout performs significantly better than setInterval.

All these glorious numbers are essentially telling us: if you want a piece of code to execute immediately, do not use timers.

//To ensure code runs immediately
//do this

//Not this
setTimeout(runAwesomeCode, 0);

For further reading, check out the additional resources below and happy coding!

Additional Resources