How Do I Include One JavaScript File in Another?
Including one JavaScript file in another is not as straightforward as using @import in CSS. Before ES6, various approaches were utilized to address this issue.
ES6 Modules
Since 2015 (ES6), JavaScript introduced ES6 modules to import modules in Node.js. Most modern browsers also support this standard. ES6 modules use the following syntax:
export function hello() {
return "Hello";
}
import { hello } from './module.js';
let val = hello(); // val is "Hello";
Node.js require
Node.js uses the older CJS module style, which is based on the module.exports/require system:
// mymodule.js
module.exports = {
hello: function() {
return "Hello";
}
}
// server.js
const myModule = require('./mymodule');
let val = myModule.hello(); // val is "Hello"
Other Browser Loading Methods
In addition to ES6 modules, browsers also provide various other options for loading external JavaScript contents:
- AJAX Loading: Use an AJAX call to load an additional script and then run it using eval.
- Fetch Loading: Load scripts using a fetch call and promises to control execution order.
- jQuery Loading: Use the jQuery library to load scripts with a single line of code.
- Dynamic Script Loading: Add a script tag with the script URL into the HTML. This allows the browser to evaluate the code.
Detecting Script Execution
When remotely loading code, modern browsers execute scripts asynchronously, so the code may not be available immediately after loading. To detect when the script has been executed, you can use:
- Event Handlers: Bind the onload or onreadystatechange events of the script tag to a callback function.
- Source Code Merge/Preprocessing: Use build/transpilation tools like Parcel, Webpack, or Babel to combine and process JavaScript files before they are loaded into the browser.
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