Exports, Module & Require

CommonJS Module System

Node.js follows the CommonJS module system, and the builtin require function is the easiest way to include modules that exist in separate files. The basic functionality of require is that it reads a JavaScript file, executes the file, and then proceeds to return the exports object. NodeJs explanation (opens in a new tab)

module.exports

By default, each module we create in NodeJs has a module object available within it, that represents the current module. One important property of this object is the .exports property, the value of which is returned when the module is required into another file.

module.exports default value is an empty object ({})

// file1.js
console.log(module.exports); // {}

require

require is a function that takes a single argument of the path to the file that is to be imported. (It can also take a 'module name', but more on that another day!)

// file2.js
const mystery = require('./file1.js');
console.log(mystery); // {}

The following example shows how module.exports can be reassigned a new value, and that value is what will be returned when the module is required into another file.

// file1.js
const name = 'phil';
module.exports = name;
 
// file2.js
const mystery = require('./file1.js');
console.log(mystery); // 'phil'

Note: The variable created in file2 from the return value of require is a brand new variable that can be named anything the developer wants, though it will often make sense to keep the same variable name across the different files

Exporting non-primitive types

Exporting objects

module.exports can be assigned any JavaScript value (not only primitives, as in the above example), and this becomes extremely useful when wanting to export multiple values from a single file inside an object. Consider the following example:

// file1.js
const name1 = 'phil';
const name2 = 'izzi';
const multipleNames = { name1: name1, name2: name2 };
 
module.exports = multipleNames;
 
// file2.js
const multipleNames = require('./file1.js');
 
console.log(multipleNames); // logs {name1: 'phil', name2: 'izzi'}
 
console.log(multipleNames.name1); // phil
console.log(multipleNames.name2); // izzi

Exporting functions

Often we may also have functions that we want to use in multiple files, and exporting and requiring those functions is the easiest way to do that:

// file1.js
const sayHello = function () {
  console.log('hello');
};
 
module.exports = sayHello;
 
// file2.js
const sayHello = require('./file1.js');
console.log(sayHello); // [Function sayHello]
// To get the 'hello' console.log, we have to invoke the required in function
sayHello(); // 'hello'

This can be done in conjunction with the previous example to export multiple objects from the same file:

// file1.js
const sayHello = function () {
  console.log('hello');
};
 
sayGoodbye = function () {
  console.log('goodbye');
};
 
module.exports = { sayHello: sayHello, sayGoodbye: sayGoodbye };
 
// file2.js
const functionsObj = require('./file1.js');
console.log(functionsObj); /*
  {
    sayHello: [Function sayHello],
    sayGoodbye: [Function sayGoodbye]
  }
*/
functionsObj.sayHello(); // 'hello'
functionsObj.sayGoodbye(); // 'goodbye'

Object short-hand property names

We can use short-hand when declaring objects if you want the new key to be the same as the variable name.

// file1.js
 
const sayHello = function () {
  console.log('hello');
};
 
sayGoodbye = function () {
  console.log('goodbye');
};
 
module.exports = { sayHello, sayGoodbye };
// equivalent to { sayHello: sayHello, sayGoodbye: sayGoodbye };