How do I include a JavaScript file inside another JavaScript file, similar to @import
in CSS?
JavaScript File Inclusion – How to Include a JavaScript File in Another JavaScript File
fileimportincludejavascript
fileimportincludejavascript
How do I include a JavaScript file inside another JavaScript file, similar to @import
in CSS?
Best Answer
The old versions of JavaScript had no import, include, or require, so many different approaches to this problem have been developed.
But since 2015 (ES6), JavaScript has had the ES6 modules standard to import modules in Node.js, which is also supported by most modern browsers.
For compatibility with older browsers, build tools like Webpack and Rollup and/or transpilation tools like Babel can be used.
ES6 Modules
ECMAScript (ES6) modules have been supported in Node.js since v8.5, with the
--experimental-modules
flag, and since at least Node.js v13.8.0 without the flag. To enable "ESM" (vs. Node.js's previous CommonJS-style module system ["CJS"]) you either use"type": "module"
inpackage.json
or give the files the extension.mjs
. (Similarly, modules written with Node.js's previous CJS module can be named.cjs
if your default is ESM.)Using
package.json
:Then
module.js
:Then
main.js
:Using
.mjs
, you'd havemodule.mjs
:Then
main.mjs
:ECMAScript modules in browsers
Browsers have had support for loading ECMAScript modules directly (no tools like Webpack required) since Safari 10.1, Chrome 61, Firefox 60, and Edge 16. Check the current support at caniuse. There is no need to use Node.js'
.mjs
extension; browsers completely ignore file extensions on modules/scripts.Read more at https://jakearchibald.com/2017/es-modules-in-browsers/
Dynamic imports in browsers
Dynamic imports let the script load other scripts as needed:
Read more at https://developers.google.com/web/updates/2017/11/dynamic-import
Node.js require
The older CJS module style, still widely used in Node.js, is the
module.exports
/require
system.There are other ways for JavaScript to include external JavaScript contents in browsers that do not require preprocessing.
AJAX Loading
You could load an additional script with an AJAX call and then use
eval
to run it. This is the most straightforward way, but it is limited to your domain because of the JavaScript sandbox security model. Usingeval
also opens the door to bugs, hacks and security issues.Fetch Loading
Like Dynamic Imports you can load one or many scripts with a
fetch
call using promises to control order of execution for script dependencies using the Fetch Inject library:jQuery Loading
The jQuery library provides loading functionality in one line:
Dynamic Script Loading
You could add a script tag with the script URL into the HTML. To avoid the overhead of jQuery, this is an ideal solution.
The script can even reside on a different server. Furthermore, the browser evaluates the code. The
<script>
tag can be injected into either the web page<head>
, or inserted just before the closing</body>
tag.Here is an example of how this could work:
This function will add a new
<script>
tag to the end of the head section of the page, where thesrc
attribute is set to the URL which is given to the function as the first parameter.Both of these solutions are discussed and illustrated in JavaScript Madness: Dynamic Script Loading.
Detecting when the script has been executed
Now, there is a big issue you must know about. Doing that implies that you remotely load the code. Modern web browsers will load the file and keep executing your current script because they load everything asynchronously to improve performance. (This applies to both the jQuery method and the manual dynamic script loading method.)
It means that if you use these tricks directly, you won't be able to use your newly loaded code the next line after you asked it to be loaded, because it will be still loading.
For example:
my_lovely_script.js
containsMySuperObject
:Then you reload the page hitting F5. And it works! Confusing...
So what to do about it ?
Well, you can use the hack the author suggests in the link I gave you. In summary, for people in a hurry, he uses an event to run a callback function when the script is loaded. So you can put all the code using the remote library in the callback function. For example:
Then you write the code you want to use AFTER the script is loaded in a lambda function:
Then you run all that:
Note that the script may execute after the DOM has loaded, or before, depending on the browser and whether you included the line
script.async = false;
. There's a great article on Javascript loading in general which discusses this.Source Code Merge/Preprocessing
As mentioned at the top of this answer, many developers use build/transpilation tool(s) like Parcel, Webpack, or Babel in their projects, allowing them to use upcoming JavaScript syntax, provide backward compatibility for older browsers, combine files, minify, perform code splitting etc.