CommonJS Modules

Let's look at the first type of module syntax called CommonJS. This is an older syntax that is used in Node.js. You won't use this much on the front-end, but if you plan on being a full-stack or back-end developer, you'll need to know it.

Let's create an app.js file and a utils.js file. Let's say that the app file is the main entry point and utils is just some extra utility functions that we want to be able to use in multiple files.

In utils.js, I am going to create a function that will capitalize the first letter of every word.

function capitalizeWords(str) {
  return str
    .toLowerCase()
    .split(' ')
    .map((word) => word[0].toUpperCase() + word.substr(1))
    .join(' ');
}

I want to be able to import this into the app.js file and use it. In order to do that, we need to export it. We do that by using the module.exports object. We can then set it equal to the function that we want to export.

module.exports = capitalizeWords;

Now, I can import it into the app.js file. In order to do that, we use the require() function. We pass in the path to the file that we want to import. We can then store it in a variable and use it.

const capitalizeWords = require('./utils');

console.log(capitalizeFirst('hello world')); // Hello World

As you can see, we can import the function and use it in the app.js file.

Exporting Multiple functions

In many cases, you will want to export more than one thing from a file. Let's create another function in utils.js.

function makeMoney(amount) {
  return `$${amount}`;
}

To export both functions, we can use the module.exports object again. We can set it equal to an object with the functions as properties.

module.exports = {
  capitalizeWords,
  makeMoney,
};

Now when we import it in the app.js file, we can destructure it and get both functions.

const { capitalizeWords, makeMoney } = require('./utils');

console.log(capitalizeWords('hello world')); // Hello World
console.log(makeMoney(100)); // $100

Exporting Classes

Functions are not the only thing that we can import/export. Let's create a file named Person.js and create a class in it.

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  greet() {
    console.log(`Hello, my name is ${this.name} and I am ${this.age}`);
  }
}

Now, let's export it

module.exports = Person;

Now we can use it in the app.js file.

const Person = require('./Person');

const person1 = new Person('John', 30);
person1.greet(); // Hello, my name is John and I am 30

Remember, this is only for Node.js. You won't be able to use this on the front-end, at least without tooling.