Node Modules

Creating, Exporting, and Importing Modules

Node's module system allows code written in one file to be exported, and then imported into other files. By importing a module (i.e. a specified section of code), we can then use that code as if it actually were written in the file we imported it to.

Let's try an example!

1. Create your module.

Inside the my-first-node-project folder, create a javascript file called myModule.js.

In Node, module.exports is an object that will hold the code to be exported. We can use dot-notation to add the code we want to export to this object.

Add the following code to your myModule.js file:

module.exports.beBasic = () => "That's so fetch!"

Now, our module.exports object has a key-value pair where the key is beBasic and the value is a function.

2. Import your module in index.js.

This is where the require function, specific to Node, comes into play. This function takes one argument: the path to the file that contains the module you are exporting.

In the index.js file, write the following code:

const myModule = require('./myModule.js');

console.log(myModule.beBasic());

Run index.js via the command line:

node index.js

Voila! You've successfully created and imported a module!

Let's add some more code to our module. In myModule.js, add the following code:

module.exports.beBasic = () => "That's so fetch!"

const count = () => {
    for (let i = 0; i <= 10; i++) {
        console.log(i);
    }
}

Now call this new count function from index.js:

const myModule = require('./myModule.js');

myModule.beBasic();
myModule.count();

Try running this code in the command line: node index.js

What happened? Why didn't this work?

The exported module will only contain the code that is encapsulated in the module.exports object!

How do we get our count function to run? Make this happen.

Functions aren't the only things we can export! Try adding some other types of data to your module.

Further Reading

To view a practical example of importing and exporting modules, read this article. You'll see that we can export multiple functions by assigning module.exports to an object. This is a pattern that we'll see frequently in Node.

Using Built-In Modules

It's great to have the flexibility to create our own modules, but Node supplies us with some simple built-in modules (aka core modules) that are ready for us to import and use!

Example: fs module

We will use the fs core module (it stands for "file system") to read a text file.

Create a story.txt text file inside your project directory and write a short story inside it.

Core modules just need to be imported using the require function.

Write the following code to your entry point file:

const fs = require('fs');

fs.readFile('story.txt', 'utf8', function(err, data){
    if(err) {
        console.log("There was a problem reading the file.");
    } else {
        console.log(data);
    }
});

Run index.js to read your story in the terminal!

For more on the fs module, see w3schools.

Try adding to your story using fs.write().

Exercise: HTTP core module

In this excercise, you will make a Hello World app from scratch by using the the HTTP core module to spin up an HTTP server.

HTTP Request and Response Diagram

1. Create a hello-node directory.

2. Initialize Node in this directory.

3. Create your entry point file.

4. Import the http module into your entry file. (Hint: use the require function)

5. Create an http server that listens to port 8000 and writes Hello, World! to the client. (Hint: look up the core http module on w3schools)
6. Run the server using the command node index.js.

7. Check to see that your program is working by visiting localhost:8000 in your browser.

SOLUTION

const http = require('http');

http.createServer((req, res) => { 
   res.write('Hello, World!');
   res.end();
})
.listen(8000);