JavaScript Performance Tips

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·Sep 22, 2022·

4 min read

JavaScript Performance Tips

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Table of contents

  • Tip 1
  • Tip 2
  • Tip 3
  • Tip 4

JavaScript, like any language, requires us to be judicious in the use of certain language features. Overuse of some features can decrease performance, while some techniques can be used to increase performance.

Tip 1

  • Avoid try/catch in performance-critical functions

Some JavaScript engines (for example, the current version of Node.js and older versions of Chrome before Ignition+turbofan) don't run the optimizer on functions that contain a try/catch block. If you need to handle exceptions in performance-critical code, it can be faster in some cases to keep the try/catch in a separate function.

function myPerformanceCriticalFunction() {
 try {
 // do complex calculations here
 } catch (e) {

Tip 2

  • Limit DOM Updates

A common mistake seen in JavaScript when run in a browser environment is updating the DOM more often than necessary. The issue here is that every update in the DOM interface causes the browser to re-render the screen. If an update changes the layout of an element in the page, the entire page layout needs to be re-computed, and this is very performance-heavy even in the simplest of cases. The process of re-drawing a page is known as reflow and can cause a browser to run slowly or even become unresponsive.

Consider the following document containing a


    <!DOCTYPE html>
     <ul id="list"></ul>

    We add 5000 items to the list looping 5000 times (you can try this with a larger number on a powerful computer to increase the effect).

    var list = document.getElementById("list");
    for(var i = 1; i <= 5000; i++) {
     list.innerHTML += `<li>item ${i}</li>`; // update 5000 times

    Tip 3

    • Benchmarking your code - measuring execution time

    Most performance tips are very dependent of the current state of JS engines and are expected to be only relevant at a given time. The fundamental law of performance optimization is that you must first measure before trying to optimize, and measure again after a presumed optimization. To measure code execution time, you can use different time measurement tools like:

    1. Performance interface that represents timing-related performance information for the given page (only available in browsers).
    let startTime, endTime;
    function myFunction() {
     //Slow code you want to measure
    //Get the start time
    startTime =;
    //Call the time-consuming function
    //Get the end time
    endTime =;
    //The difference is how many milliseconds it took to call myFunction()
    console.debug('Elapsed time:', (endTime - startTime));
    The result in console will look something like this:
    Elapsed time: 0.10000000009313226
    1. process.hrtime on Node.js gives you timing information as [seconds, nanoseconds] tuples. Called without argument it returns an arbitrary time but called with a previously returned value as argument it returns the difference between the two executions.
    2. Consoletimers console.time("labelName") starts a timer you can use to track how long an operation takes. You give each timer a unique label name, and may have up to 10,000 timers running on a given page. When you call console.timeEnd("labelName") with the same name, the browser will finish the timer for given name and output the time in milliseconds, that elapsed since the timer was started. The strings passed to time() and timeEnd() must match otherwise the timer will not finish.
    3. function returns current Timestamp in milliseconds, which is a Number representation of time since 1 January 1970 00:00:00 UTC until now. The method now() is a static method of Date, therefore you always use it as
    let t0 =; //stores current Timestamp in milliseconds since 1 January 1970 00:00:00 UTC
    let arr = []; //store empty array
    for (let i = 0; i < 1000000; i++) { //1 million iterations
     arr.push(i); //push current i value
    console.log( - t0); //print elapsed time between stored t0 and now

    Tip 4

    • Initializing object properties with null

    The best way to make an object predictable is to define a whole structure in a constructor. So if you're going to add some extra properties after object creation, define them in a constructor with null. This will help the optimizer to predict object behavior for its whole life cycle. However, all compilers have different optimizers and the performance increase can be different, but overall it's good practice to define all properties in a constructor, even when their value is not yet known.

    That's it for Today

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