Implement dynamic loading of CSS and JS files through JavaScript

Basic concepts and functions descriptions

The meaning of dynamic loading

Dynamic loading refers to the process of loading resource files on demand based on user operations or specific conditions after page initialization. Doing so can significantly reduce the time of initial loading, as all possible resources are not required to be loaded from the beginning.

The role of CSS and JS files

CSS (Cascade Style Sheet): Used to control the expression style of web pages, such as color, layout, font size, etc.
JavaScript: A widely used programming language, mainly used to implement interactive functions on web pages.

How to dynamically load CSS files

Example 1: Dynamically add CSS files using the <link> tag

function loadCSS(url) {
    const link = ('link');
     = 'stylesheet';
     = url;
    // Prevent cache     += (('?') &gt;= 0 ? '&amp;' : '?') + 'noCache=' + new Date().getTime();
    (link);
}

// Use exampleloadCSS('path/to/your/');

This method utilizes HTML<link>Tags to load CSS files asynchronously. Create and insert dynamically<link>Elements, we can easily load new stylesheets at runtime.

Example 2: Listening to load events

function loadCSSWithCallback(url, callback) {
    const link = ('link');
     = 'stylesheet';
     = url;
     = function() {
        ('CSS file loaded.');
        if (typeof callback === 'function') {
            callback();
        }
    };
     = function() {
        ('Failed to load CSS file.');
    };
    (link);
}

// Use exampleloadCSSWithCallback('path/to/your/', function() {
    ('CSS is now ready to use.');
});

In this example, we have added the handling of load completion and load failure, and we can perform further operations through the callback function.

Methods to dynamically load JavaScript files

Example 3: Dynamically load JS files using <script> tags

function loadScript(url, callback) {
    const script = ('script');
     = url;
     = true; // Asynchronous loading scripts     = function() {
        ('Script file loaded.');
        if (typeof callback === 'function') {
            callback();
        }
    };
     = function() {
        ('Failed to load script file.');
    };
    (script);
}

// Use exampleloadScript('path/to/your/', function() {
    ('Script is now ready to use.');
});

Here is how to create it<script>Elements to dynamically load JavaScript files. set upasyncProperties allow scripts to be loaded asynchronously without blocking page rendering.

Example 4: Handling Cross-domain Loading

function loadCrossDomainScript(url, callback) {
    const script = ('script');
     = url;
     = 'anonymous'; // Set cross-domain properties     = function() {
        ('Cross-domain script file loaded.');
        if (typeof callback === 'function') {
            callback();
        }
    };
     = function() {
        ('Failed to load cross-domain script file.');
    };
    (script);
}

// Use exampleloadCrossDomainScript('/path/to/', function() {
    ('Cross-domain script is now ready to use.');
});

For cross-domain requests, we need to setcrossOriginproperties to ensure the request can be successful.

Ideas for using functions from different angles

Load on demand

Decide whether to load certain resources based on the user's behavior or the status of the page. For example, the corresponding JS or CSS file is loaded only when the user clicks a button, which can avoid unnecessary resource consumption.

Performance optimization

By delaying loading non-critical resources, you can speed up the loading of main content on your page, thereby improving the user experience. For example, asynchronously load ad-related JavaScript code at the bottom of the page.

Loading under specific conditions

Selectively load specific resource files for different devices or browser versions. For example, provide additional compatibility scripts for older browsers.

Some tips in actual work

Preload prompt: For some important resources, you can use them in HTML headers<link rel="preload">Inform the browser in advance that these resources will be used so that the download will start as soon as possible.
Resource merger: Reduce the number of HTTP requests, and improve loading efficiency by combining multiple small files into one large file.
Lazy loading: For media resources such as images, lazy loading technology can be used, that is, the image is loaded when the user scrolls near the element, rather than loading all the images from the beginning.
Error handling: Always add error handling logic to load operations to ensure that even if a resource fails to load, it will not affect the normal operation of the entire page.

Dynamic loading of CSS and JS files is one of the effective means to improve the performance of web applications. By rationally applying the above technologies and strategies, developers can build more efficient and responsive applications. Hopefully the examples and suggestions provided in this article will help you achieve a better user experience in your project.

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