How to include JS file to another JS file?

Tine JavaScript tutorial on a sophisticated topic with code examples
30 August 2017   165123

Hype.codes team made a research and found several ways to solve this issue.

The old versions of JavaScript had no import, include, or require, so many different approaches to this problem have been developed.

But recent versions of JavaScript have standards like ES6 modules to import modules, although this is not supported yet by most browsers. Many people using modules with browser applications use build and/or transpilation tools to make it practical to use new syntax with features like modules.

ES6 Modules

Note that currently, browser support for ES6 Modules is not particularly great, but it is on it's way. They are supported (but behind flags) in Chrome 60, Firefox 54 and MS Edge 15, with only Safari 10.1 providing support without flags.

Thus, you will currently still need to use build and/or transpilation tools to valid JavaScript that will run in without any requirement for the user to use those browser versions or enable any flags.

Once ES6 Modules are commonplace, here is how you would go about using them:

// module.js
export function hello() {
  return "Hello";
}

// main.js
import {hello} from 'module'; // or './module'
let val = hello(); // val is "Hello";

Node.js

Node.js is currently using a module.exports/require system. You can use babel to transpile if you want the import syntax.

// mymodule.js
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.

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. Using eval also opens the door to bugs, hacks and security issues.

jQuery Loading

The jQuery library provides loading functionality in one line:

$.getScript("my_lovely_script.js", function(){

   alert("Script loaded but not necessarily executed.");

});

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:

function dynamicallyLoadScript(url) {
    var script = document.createElement("script"); //Make a script DOM node
    script.src = url; //Set it's 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 end of the head section of the page, where the srcattribute 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.

How to detect executed script?

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! 

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