JavaScript Importing and Exporting Modules

JavaScript Modules are basically libraries which are included in the given program. They are used for connecting two JavaScript programs together to call the functions written in one program without writing the body of the functions itself in another program. 

Importing a library: It means include a library in a program so that use the function is defined in that library. For this, use 'require' function in which pass the library name with its relative path. 

Example: Suppose a library is made in the same folder with file name library.js, then include the file by using require function:

const lib = require('./library') 

which will return a reference to that library. Now if there is an area function defined in the library, then use it as lib.area(). 

Exporting a library: There is a special object in JavaScript called module.exports. When some program include or import this module (program), this object will be exposed. Therefore, all those functions that need to be exposed or need to be available so that it can use in some other file, defined in module.exports. 

Example: In this example, we will write two different programs and then see how to use functions defined in the library (Module) in a given program. We will define two simple functions in the library for calculating and printing area and perimeter of a rectangle when provided with length and breadth. Then export the functions so that other programs can import them if needed and can use them.

Exporting Module Example: library.js

<script>
    let area = function (length, breadth) {
        let a = length * breadth;
        console.log('Area of the rectangle is ' + a + ' square unit');
    }
    
    let perimeter = function (length, breadth) {
        let p = 2 * (length + breadth);
        console.log('Perimeter of the rectangle is ' + p + ' unit');
    }
    
    module.exports = {
        area,
        perimeter
    }
</script>

Importing Module Example

For importing any module, use a function called 'require' which takes in the module name, and if it is a user-defined module then its relative path as an argument and returns its reference. 

The script.js contains the above JavaScript module (library.js). 

Script.js 

<script>
    const lib = require('./library'); 
    // " ./ " is used if both the files are in the same folder.
    let length = 10;
    let breadth = 5;

    lib.area(length, breadth);
    lib.perimeter(length, breadth);
</script>

Output:

Area of the rectangle is 50 square unit
Perimeter of the rectangle is 30 unit

To learn about new methods for exporting and importing in ES6. Click here

Note: To run the script first make both files in the same folder and then run script.js using NodeJs interpreter in the terminal.