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.
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