How to include a JavaScript file in another JavaScript file ?

Difficulty Level : Expert
Last Updated : 31 Jul, 2019

In native JavaScript before ES6 Modules 2015 has been introduced had no import, include, or require, functionalities. Before that, we can load a JavaScript file into another JavaScript file using a script tag inside the DOM that script will be downloaded and executed immediately.

Now after the invention of ES6 modules there are so many different approaches to solve this problem have been developed and discussed below.

ES6 Modules: ECMAScript (ES6) modules have been supported in Node.js since v8.5. In this module, we define exported functions in one file and import them in another example.
There are two popular way to call a JavaScript file from another function those are listed below:

Ajax Techniques
Concatenate files

Ajax Techniques Example:

External JavaScript file named as “main.js”
// This alert will export in the main file
alert("Hello Geeks")

Main file: This file will import the above “main.js” file

<!DOCTYPE html>
<html>
  
<head>
    <title>
        Calling JavaScript file from
        another JavaScript file
    </title>
      
    <script type="text/javascript">
        var script = document.createElement('script');
          
        script.src = 
"https://media.geeksforgeeks.org/wp-content/uploads/20190704153043/main.js";
          
        document.head.appendChild(script)
    </script>
</head>
  
<body>
</body>
  
</html>

Output:

Concatenate files Example: Here importing multiple JavaScript files into a single JavaScript file and calling that master JavaScript file from a function.

External JavaScript file named as “main.js”
// This alert will export in the main file
alert("Hello Geeks")
External JavaScript file “second.js”
// This alert will export in the main file
alert("Welcome to Geeksforgeeks")

External JavaScript file “master.js”

function include(file) {
  
  var script  = document.createElement('script');
  script.src  = file;
  script.type = 'text/javascript';
  script.defer = true;
  
  document.getElementsByTagName('head').item(0).appendChild(script);
  
}
  
/* Include Many js files */
include('https://media.geeksforgeeks.org/wp-content/uploads/20190704153043/main.js');
include('https://media.geeksforgeeks.org/wp-content/uploads/20190704162640/second.js');

Main file: This file will import the above “master.js” file

<!DOCTYPE html>
<html>
  
<head>
    <title>
        Calling JavaScript file from
        another JavaScript file
    </title>
      
    <script type="text/javascript"
src="https://media.geeksforgeeks.org/wp-content/uploads/20190704162730/master.js">
    </script>
</head>
  
<body>
</body>
</html>           