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How do I include a JavaScript file in another JavaScript file?

How do I include a JavaScript file in another JavaScript file?

David Y.

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The Problem

How do I include a JavaScript file in another JavaScript file?

The Solution

Modern JavaScript provides built-in support for including external files through its module syntax. This was introduced as a feature of the core language with ECMAScript 6 in 2015. Moduling systems such as CommonJS and RequireJS were created before this language feature was introduced, and are still in common use today. These systems are, however, not directly supported by browsers.

All modern browsers support JavaScript modules, making them the easiest way to split JavaScript up between files. Let’s work through a quick example. Consider a file tree like that below:

Click to Copy
├── index.html
├── calculations.js
└── script.js

index.html is the homepage of our website and script.js contains its JavaScript. calculations.js is a JavaScript module, containing utility functions we would like to use in script.js.

The contents of calculations.js will look like this:

Click to Copy
// calculations.js
export function calculateMarkup(costPrice, salePrice) {
  return (salePrice - costPrice) / costPrice;
}

Note the use of export: this marks functions that can be imported into other files.

We would then import and use our the calculateMarkup function in script.js like this:

Click to Copy
// script.js
import { calculateMarkup } from "./calculations.js";

export function showAppleMarkupPercentage() {
  const div = document.createElement("div");
  div.textContent = `The markup on apples is ${calculateMarkup(1, 2) * 100}%.`;
  document.body.appendChild(div);
}

import can only be used inside modules, so the JavaScript interpreter will expect both calculations.js and script.js to be modules. If we want to use functions from script.js in index.html, we will have to mark individual <script> tags as type=module and import them:

Click to Copy
<!-- index.html -->
<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Apple Store</title>
  </head>
  <body>
    <script type="module">
      import { showAppleMarkupPercentage } from "./script.js"
      showAppleMarkupPercentage();
    </script>
  </body>
</html>

A few additional notes on using JavaScript modules:

  1. Module filenames must be preceded with a directory specified. If the module is in the same directory as the file importing it, use ./.
  2. JavaScript modules cannot be loaded using file:// URLs. Local testing of applications using this functionality must be done with a local web server.
  3. Module functions are only available in the file they are imported into, not the global namespace.
  4. Modules automatically use strict mode.
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