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Introduction

Web performance is crucial for providing a great user experience, and loading JavaScript is one of the major factors that can slow down a website. Fortunately, JavaScript provides two ways to load asynchronous JavaScript files: async and defer. Using these techniques can speed up your website, improve your SEO, and enhance your user experience.

In this article, we'll dive into the differences between async and defer, and how to use them correctly to boost web page performance.

What are Async and Defer?

Async and defer are two attributes for the <script> tag. They tell the browser how to fetch and execute the JavaScript file.

When the browser encounters the <script> tag with the async attribute, it will start downloading the script file and continue parsing the rest of the page. The script will execute as soon as it is downloaded, even if the page hasn't finished rendering yet.

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

On the other hand, when the browser encounters the <script> tag with the defer attribute, it will continue parsing the HTML document until it reaches the end. Then it will start downloading the script file and execute it after the HTML document is parsed.

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

Choosing Between Async and Defer

Choosing between async and defer depends on your use case. Here are some things to consider:

• If the script file is essential for rendering the page, use the defer attribute.

• If the script file is not essential for rendering the page and it doesn't depend on any other scripts, use the async attribute.

• If the script file depends on other scripts or the page's content, you should use the defer attribute.

Best Practices

Here are some best practices to keep in mind when using async and defer:

• Avoid using async and defer on the same script file.

• Don't use async or defer on scripts that write to the DOM.

• Place the async scripts in the head of the HTML document.

• Place the defer scripts before the closing </body> tag.

• Always test and validate your scripts to ensure they're working as expected.

Conclusion

In conclusion, using the async and defer attributes correctly can reduce the load time of your web pages, improve your SEO, and enhance your user experience. By choosing the right technique based on your use case and following the best practices, you can boost your web page performance and keep your users happy.