How do I include a JavaScript file in another JavaScript file?

Is there something in JavaScript similar to @import in CSS that allows you to include a JavaScript file inside another JavaScript file?

Solution:

JavaScript has no import, include, or require. (Update 2015: it does now with ES6 modules) There are other ways for JavaScript to include external JavaScript contents, though.

Ajax Loading

Load an additional script with an Ajax call and then use eval. 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 and hacks.

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

Add a script tag with the script URL in 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.
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 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.getElementsByTagName('head')[0];
    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 what ever 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.

Source Code Merge

Another solution is to combine the two files into a single file. This can be used with minification to produce a single, minimally sized JavaScript file to include as normal.


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