JavaScript Performance

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JavaScript Performance


How to speed up your JavaScript code.

Reduce Activity in Loops
Loops are often used in programming.
Each statement in a loop, including the for statement, is executed for each iteration of the
loop.
Statements or assignments that can be placed outside the loop will make the
loop run faster.

Bad:

for (let i = 0; i < arr.length; i++) {



Better Code:

let l = arr.length;
for (let i = 0; i < l; i++) {

The bad code accesses the length property of an array each time the loop is
iterated.
The better code accesses the length property outside the loop and makes the
loop run faster.

Reduce DOM Access
Accessing the HTML DOM is very slow, compared to other JavaScript statements.
If you expect to access a DOM element several times, access it once, and use
it as a local variable:

Example

const obj = document.getElementById("demo");obj.innerHTML = "Hello";

Try it Yourself »








Reduce DOM Size
Keep the number of elements in the HTML DOM small.
This will always
improve page loading, and speed up rendering (page display), especially on smaller devices.
Every attempt to search the DOM (like getElementsByTagName) will benefit
from a smaller DOM.

Avoid Unnecessary Variables
Don't create new variables if you don't plan to save values.
Often you can replace code like this:


let fullName = firstName + " " + lastName;
document.getElementById("demo").innerHTML = fullName;


With this:


document.getElementById("demo").innerHTML = firstName + " " + lastName;



Delay JavaScript Loading
Putting your scripts at the bottom of the page body lets the browser load the
page first.
While a script is downloading, the browser will not start any other
downloads. In addition all parsing and rendering activity might be blocked.

The HTTP specification defines that browsers should not download more than
two components in parallel.

An alternative is to use defer="true" in the script tag. The
defer attribute specifies that the script should be executed after the page has
finished parsing, but it only works for external scripts.
If possible, you can add your script to the page by code, after the page has loaded:

Example

<script>window.onload = function() { 
const element = document.createElement("script"); 
element.src = "myScript.js"; 
document.body.appendChild(element);};</script>


Avoid Using with
Avoid using the with keyword. It has a negative effect on
speed. It also clutters up JavaScript scopes.
The with keyword is not allowed in strict mode.












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Reference: https://www.w3schools.com/js/js_performance.asp