How do I include a JavaScript file in another JavaScript file?
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:
├── 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:
// 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:
// 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:
<!-- 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:
./
.file://
URLs. Local testing of applications using this functionality must be done with a local web server.Get actionable, code-level insights to resolve JavaScript performance bottlenecks and errors.
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