Optimize your code: load code at the right time.

<script src="magic.js">

We have three steps

Loading
Parsing
Executing

Loading

When the browser sees the script tag it asks your server for the script named magic.js. Your server sends magic.js to the browser.

Parsing

The browser then needs to analyze the code.

Executing

Once the code has been parsed the browser runs it.

All of this (Loading, Parsing, and Executing) takes time so we aim to solve this problem.

Solving the Problem

We have three levels of goodness (three techniques to optimize website performance by loading JavaScript as late as possible.)

Good
Better
Extreme

Good

Don't let scripts block content as it doesn't make the browser load, parse, and execute scripts before visible content loads.

We take large scripts out of the head and put them in the body.

Better

When you can load those scripts while visible content is loading and tell the browser it's ok to parse and execute them later.

async

Tells the browser that the script can be executed asynchronously. This means while the scripts being fetched keep parsing in the HTML. Once loads stop everything and execute it.
defer

Not only does the browser keep parsing in the HTML while loading the scripts, but it waits to execute the scripts until after the document's been parsed.

Extreme

We are going to tell the browser to wait to load scripts until the visible content is complete.

   window.addEventListener("load", function(){
    const script = document.createElement("script");
    script.src = "script.js";
    document.body.appendChild(script);
});

Compare between defer, async, and normal script

OovG7

In the end, I hope you have known the Three techniques to optimize website performance by loading JavaScript as late as possible.

JavaScript Loading Scripts