How Can I Include JavaScript Files in Other JavaScript Files?

Including JavaScript Files in Other JavaScript Files

In JavaScript, importing modules is different from CSS's @import. Early versions of JavaScript lacked import, include, or require functions, leading to various approaches for including JavaScript files in others. However, since ES6 (2015), JavaScript has introduced the ES6 modules standard to import modules into Node.js and most modern browsers.

ES6 Modules

ES6 modules use the following syntax:

import { function } from './module.js';
// or
import * as module from './module.js';

ECMAScript Modules in Browsers

Browsers support ES6 module loading directly, without build tools like Webpack. Use the following syntax:

<script type="module">
  import { function } from './module.js';
</script>

Node.js require

Node.js uses the CJS module style:

const module = require('./module.js');

Other Methods

Besides ES6 modules and Node.js require, other ways to include JavaScript files in browsers include:

• AJAX Loading: Use an AJAX call to load the script and eval to run it. Limited to the same domain due to security.
• Fetch Loading: Use fetch with promises to control the execution order of scripts.
• jQuery Loading: Use jQuery to load scripts in a single line.
• Dynamic Script Loading: Create a script tag with the URL and append it to the HTML.

Detecting Script Execution

Including JavaScript files remotely means asynchronous loading. To ensure that the loaded code is available immediately, use the following technique:

function loadScript(url, callback) {
  // Create script tag
  var script = document.createElement('script');
  script.type = 'text/javascript';
  script.src = url;

  // Bind event to callback function
  script.onreadystatechange = callback;
  script.onload = callback;

  // Append to HTML
  document.head.appendChild(script);
}

Source Code Merge/Preprocessing

Build tools like Parcel, Webpack, and Babel can be used to merge source codes, provide backward compatibility, and minify files.

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