How to use JavaScript to optimize page loading

With the continuous development of the Internet, the loading speed of web pages has attracted more and more attention. For a web page, the page loading time determines the user experience. If the page loading time is too long, users will lose their patience, which will directly affect the website's traffic and user reputation. Therefore, in the actual web development process, how to optimize the page loading speed is an important issue. This article will introduce how to use JavaScript to optimize page loading.

1. Page loading process

Before discussing how to optimize page loading, you first need to understand the loading process of web pages. Generally speaking, when a user visits a web page, it is loaded in the following order:

1. Establishing a connection: The user's browser sends a request to the server, and the server establishes a connection after receiving the request.
2. Send a request: The browser sends a request to the server to request all required resources, including HTML, CSS, JavaScript, images, etc.
3. Accept response: The server sends the requested resources to the browser, and the browser starts parsing the HTML file and sends a new request to obtain resources such as CSS and JavaScript.
4. Parse HTML: The browser parses HTML files and builds DOM trees, CSSOM trees, and rendering trees.
5. Load script: The browser downloads the JavaScript file and executes the script.
6. Rendering page: Based on the DOM tree, CSSOM tree and JavaScript script, the final page is rendered and displayed to the user.

2. JavaScript optimized page loading

In the above page loading process, the loading and execution of JavaScript scripts are the links that take a long time. Therefore, in web development, how to optimize the loading and execution of JavaScript to improve the loading speed of the page has become an urgent problem to be solved.

The following are related tips for JavaScript optimization:

1. Use defer and async attributes

In HTML, you can use defer and async in the script tag Attributes to control the loading and execution of JavaScript scripts:

(1) defer attribute: Indicates that the loading and parsing of the script is performed asynchronously, that is, the download will not block the parsing of the HTML, but will wait until the HTML document is parsed. Then execute the script.

(2) async attribute: Indicates that the loading and parsing of the script are performed asynchronously, but this process will block the rendering of the page. That is, during the page rendering process, the script may not be downloaded yet, which may Will cause the page to flicker.

Using the defer attribute can solve the blocking problem of JavaScript scripts without affecting page rendering. The use of the async attribute is to speed up the download and execution of scripts, but it may affect the user experience.

2. Reduce the size of JavaScript as much as possible

The larger the size of the JavaScript script, the longer it will take to download and parse. Therefore, when writing JavaScript code, you should try to reduce the size of the code as much as possible. Specifically, you can take the following methods:

(1) Compress JavaScript files: You can use some tools (such as uglifyjs, etc.) to compress JavaScript files and remove some useless information such as comments, spaces, and newlines.

(2) Minify JavaScript files: Some commonly used JavaScript libraries (such as jQuery, etc.) can be reduced to only the parts that need to be used.

(3) Avoid redundant code: When writing JavaScript code, avoid the existence of redundant code as much as possible. You can check problems in the code through some tools (such as JSLint, JSHint, etc.).

3. Request to minimize JavaScript code

When loading a JavaScript script, each file will increase the number of HTTP requests. Therefore, the request to minimize JavaScript code is An important aspect of JavaScript optimization. You can take the following methods:

(1) Merge JavaScript files: Merge multiple similar JavaScript files into one file, which can reduce HTTP requests and improve performance.

(2) Place the JavaScript file at the bottom: This way, the JavaScript script can be loaded after the page content is loaded, improving the user experience.

(3) Use CDN: CDN (Content Delivery Network) is a distributed network that can cache and quickly transmit JavaScript files, thereby improving the loading speed of JavaScript.

4. Use local cache

In web development, using local cache allows JavaScript scripts to be cached in the browser for easy use next time. This reduces HTTP requests and increases the loading speed of JavaScript files. The following methods can be used:

(1) Use localStorage to store JavaScript files: You can use LocalStorageAPI to store JavaScript files in the local cache. You can read them directly from the cache the next time you use them, improving the loading speed of JavaScript files. .

(2) Use Service Worker: Service Worker is a script that runs between the browser and the network. It can cache JavaScript files locally. When used next time, it can be read directly from the cache and improve JavaScript files. loading speed.

3. Conclusion

Through the introduction of this article, we can know that JavaScript optimization is a very important issue, which involves the loading speed and user experience of web pages. When using JavaScript for page optimization, you need to understand the loading and execution process of JavaScript, minimize the size of JavaScript code, minimize requests for JavaScript code, and use local caching and other techniques to improve page loading speed and user experience.

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