As website owners, we’re always looking for ways to optimize our sites to ensure that they load as quickly as possible. One way to do this is by deferring the parsing of JavaScript until it’s needed.

When a browser loads a page, it must parse the HTML in order to understand what the page contains. This process can be slow, particularly if there is a lot of HTML to parse. If the HTML contains JavaScript, the browser must also parse that before it can execute any code.

Deferring parsing of JavaScript means that the browser can continue to process the HTML while the JavaScript is downloading. It can then execute the code once it has finished downloading and parsed it. This can help improve performance because the browser doesn’t have to wait for all of the JavaScript to download before it can start rendering anything on the page.

There are several ways to defer parsing of JavaScript. One is to use the async attribute on script tags:

<script defer src="example.js" async></script>

This tells the browser that it can download and execute example.js as soon as it’s available, without waiting for any other scripts to finish downloading. However, there’s no guarantee that example.js will be executed before other scripts on the page that don’t have the async attribute (such as inline scripts). So if your script depends on another script being executed first, you shouldn’t use async .

Another way to defer parsing of JavaScript is using an event handler like this:

window.addEventListener('DOMContentLoaded', function() { // Parse JS here });

Here we told the browser to execute the code inside the function once the HTML has been parsed. This can be useful if you have inline scripts that depend on the DOM being ready.

You can also use a tool like defer.js to automatically defer parsing of JavaScript files without having to modify your code.

Deferring parsing of JavaScript can help improve performance by allowing the browser to continue processing the HTML while the JavaScript is downloading.

It can also be used to ensure that inline scripts are executed after the DOM is ready. There are several ways to defer parsing of JavaScript, including using the async attribute on script tags and using event handlers.