DEV Community

Cover image for The Lifecycle of a JavaScript File in the Browser: Request, Load, Execute1
Mohammed Salah
Mohammed Salah

Posted on • Edited on

2

The Lifecycle of a JavaScript File in the Browser: Request, Load, Execute1

Lifecycle of a JavaScript file in the browser: request, load, and execution

In this article, we'll walk through the life of a JavaScript file and see the full version of a file request, from its download, all the way to its execution. Understanding this method can help developers improve the performance of web pages and thus the user experience.

1. Request:

When the browser encounters a script tag in an HTML document, it sends an HTTP request to the server to get the JavaScript file specified in the src attribute. This request can be synchronous or asynchronous depending on how the tag is set up.

2. Load:

Once the request is sent, the browser starts loading the JavaScript file. During this phase, the browser can load multiple files in parallel to improve performance. However, the browser must be careful in the order in which files are loaded and executed to ensure that the page functions correctly.

3. Execute:

After the JavaScript file is fully loaded, the browser starts executing it. If the tag contains an async property, the file is executed immediately after it is uploaded. If it contains the defer property, the file is executed after the entire page is fully loaded and parsed.

1 - Request: The browser sends an HTTP request to the server to obtain a JavaScript file.
2 - Load: The browser starts loading the JavaScript file after receiving the request.
3 - Execute: The browser executes the JavaScript file after it has completed loading.

  • Large or multiple JavaScript files can cause page loading delays. Therefore, it is important to use techniques such as async and defer to improve performance.

  • Developers need to be aware of how the order in which JavaScript files are loaded and executed affects the page's cross-browser compatibility.

Understanding the life cycle of a JavaScript file in a browser can help developers write more efficient code and improve user experience. By optimizing the request, load, and fulfillment process, page load time can be reduced and overall performance improved.

Heroku

Build apps, not infrastructure.

Dealing with servers, hardware, and infrastructure can take up your valuable time. Discover the benefits of Heroku, the PaaS of choice for developers since 2007.

Visit Site

Top comments (0)

Image of Timescale

Timescale – the developer's data platform for modern apps, built on PostgreSQL

Timescale Cloud is PostgreSQL optimized for speed, scale, and performance. Over 3 million IoT, AI, crypto, and dev tool apps are powered by Timescale. Try it free today! No credit card required.

Try free

👋 Kindness is contagious

Please leave a ❤️ or a friendly comment on this post if you found it helpful!

Okay