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Is there something in JavaScript similar to @import in CSS that allows you to include a JavaScript file inside another JavaScript file?
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use "var" while creating a function, then you can access that from another file. make sure both files are well connected to your project and can access each other.
You can write your JS in separate files, but when it comes to deploying, it's more efficient to minify them all into a single file.
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.
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" in package.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:
{ "type": "module" } Then module.js:
export function hello() { return "Hello"; } Then main.js:
import { hello } from './module.js'; let val = hello(); // val is "Hello"; Using .mjs, you'd have module.mjs:
export function hello() { return "Hello"; } Then main.mjs:
import { hello } from './module.mjs'; let val = hello(); // val is "Hello"; 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.
<script type="module"> import { hello } from './hello.mjs'; // Or the extension could be just `.js` hello('world'); </script> // hello.mjs -- or the extension could be just `.js` export function hello(text) { const div = document.createElement('div'); div.textContent = `Hello ${text}`; document.body.appendChild(div); } Read more at https://jakearchibald.com/2017/es-modules-in-browsers/
Dynamic imports let the script load other scripts as needed:
<script type="module"> import('hello.mjs').then(module => { module.hello('world'); }); </script> Read more at https://developers.google.com/web/updates/2017/11/dynamic-import
The older CJS module style, still widely used in Node.js, is 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" There are other ways for JavaScript to include external JavaScript contents in browsers that do not require preprocessing.
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. Using eval also opens the door to bugs, hacks and security issues.
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:
fetchInject([ 'https://cdn.jsdelivr.net/momentjs/2.17.1/moment.min.js' ]).then(() => { console.log(`Finish in less than ${moment().endOf('year').fromNow(true)}`) }) The jQuery library provides loading functionality in one line:
$.getScript("my_lovely_script.js", function() { alert("Script loaded but not necessarily executed."); }); 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:
function dynamicallyLoadScript(url) { var script = document.createElement("script"); // create a script DOM node script.src = url; // set its src to the provided URL document.head.appendChild(script); // add it to the end of the head section of the page (could change 'head' to 'body' to add it to the end of the body section instead) } This function will add a new <script> tag to the end of the head section of the page, where the src 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.
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 contains MySuperObject:
var js = document.createElement("script"); js.type = "text/javascript"; js.src = jsFilePath; document.body.appendChild(js); var s = new MySuperObject(); Error : MySuperObject is undefined 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:
function loadScript(url, callback) { // Adding the script tag to the head as suggested before var head = document.head; var script = document.createElement('script'); script.type = 'text/javascript'; script.src = url; // Then bind the event to the callback function. // There are several events for cross browser compatibility. script.onreadystatechange = callback; script.onload = callback; // Fire the loading head.appendChild(script); } Then you write the code you want to use AFTER the script is loaded in a lambda function:
var myPrettyCode = function() { // Here, do whatever you want }; Then you run all that:
loadScript("my_lovely_script.js", myPrettyCode); 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.
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.
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If anyone is looking for something more advanced, try out RequireJS. You'll get added benefits such as dependency management, better concurrency, and avoid duplication (that is, retrieving a script more than once).
You can write your JavaScript files in "modules" and then reference them as dependencies in other scripts. Or you can use RequireJS as a simple "go get this script" solution.
Example:
Define dependencies as modules:
some-dependency.js
define(['lib/dependency1', 'lib/dependency2'], function (d1, d2) { //Your actual script goes here. //The dependent scripts will be fetched if necessary. return libraryObject; //For example, jQuery object }); implementation.js is your "main" JavaScript file that depends on some-dependency.js
require(['some-dependency'], function(dependency) { //Your script goes here //some-dependency.js is fetched. //Then your script is executed }); Excerpt from the GitHub README:
RequireJS loads plain JavaScript files as well as more defined modules. It is optimized for in-browser use, including in a Web Worker, but it can be used in other JavaScript environments, like Rhino and Node. It implements the Asynchronous Module API.
RequireJS uses plain script tags to load modules/files, so it should allow for easy debugging. It can be used simply to load existing JavaScript files, so you can add it to your existing project without having to re-write your JavaScript files.
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