How to include JS file to another JS file?

Tine JavaScript tutorial on a sophisticated topic with code examples
30 August 2017   177134

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! 

Frontend News Digest 11 - 17.01

Learn about big Googles plans about cookies, TypeScript 3.8 Beta release, new Microsoft Edge built on Chromium and even more
17 January 2020   173

Greetings! I hope your week went great! Here's new frontend technologies news digest.

Learn that Google will phase out user-agent strings in Chrome, check the tips for building the apps for new, Chromium-based Microsoft Edge, check the speech of Monica Lent about building resilient frontend architecture at GOTO 2019 and other interesting articles, guides, updates, etc

Guides

  • Why should you use HSL color representation in CSS?

Learn why and (the main thing) how to sue the HSL color representation in CSS

Articles

  • Google to phase out user-agent strings in Chrome

Big company's move - Chrome plans to shift to using Client Hints instead of user-agent strings

  • Intent to experiment: Web NFC

Chrome experiments with the NFC, a lot of interesting things may appear

  • Get started building extensions for the new Microsoft Edge

If you would like to create any app for the new Microsoft browser - check this article and be advised its built on Chromium, so most existing extensions built for Chrome will work without any changes

  • Google plans to kill off third-party cookies in Chrome ‘within 2 years’

The grace period of 2 years is pretty a lot, but, an IT giant is asking advertisment industry to help create more privacy-based alternative to cookies

  • Microsoft spots malicious npm package stealing data from UNIX systems

This malicious package was active on the npm for two weeks and downloaded ‘at least 32 times’ before it was spotted by Microsoft

  • Array reduce vs Chaining vs for Loop 

Check the difference between different approaches of operating an array

Update

  • Upgrading to the new Microsoft Edge

The newest version of the Microsoft's web browser is built on Chromium, so this can really change a lot

  • Announcing TypeScript 3.8 Beta

The beta of the new version Microsoft-backed strict syntactical superset of JavaScript is available  - a good news for all TS coders

Video

  • GOTO 2019 • Building Resilient Frontend Architecture • Monica Lent