We’re now ready to create the CLI for our note taking app. First thing we’re going to do is create a new Node.js project, we can do this by using the npm cli:
npm init
The init command will create a package.json file for us on the root of the directory in which we ran the command. Don’t worry about npm and the package.json, we’ll get to those soon enough.
Next, let’s create the file where we’ll write our JavaScript for the CLI. Create a file called index.js on the root if you don’t have one already. Here in this file add a for now, just add a console.log. We’ll create the logic for allowing a user to add a new note to the app. So we need a few things:
Let’s start by creating a command we can use in our terminal. To do that, we have to register a command name in a package.json:
{
"name": "note",
"bin": {
"note": "./index.js"
}
}
Under the bin field, we can use pretty much any name we want. This name will end up being the command we use in the terminal as our CLI, so pick a good name! We’ll just use note. Notice we add the file path to that js file we created. That’s telling Node.js that we want to execute that file with the note command is ran. We need to augment that file with some hints for our machine. That’s because our computer might have many OS languages installed, like python or ruby, so it won’t know which one of these should be used to execute our program. We want to use Node.js to execute our js file. So at the top, the very first line of our JS file, we add:
#!/usr/bin/env node
This is a hashbang. A hashbang is a special comment placed at the top of a script that tells the operating system which interpreter to use when running the script. In the context of a Node.js CLI app, the hashbang tells the machine to use Node.js as the interpreter to execute the JS file.
Next, we need to install this CLI on our machine so we can test it out. We could actually install this globally like it’s a 3rd party module, but instead we’re going to create a symlink.
A symlink, or symbolic link, is a special type of file that acts as a reference to another file or directory. In the context of a Node.js CLI app, creating a symlink allows us to run the CLI from anywhere in our file system, just like a globally installed third-party module. This means that we can run our note command from any directory on our machine, without having to navigate to the directory where the CLI code is stored. Creating a symlink is done using the npm link command, which creates a symlink from the globally installed npm package to the locally stored code. So run the command:
npm link
We should not be able to run the note command in our terminal and see our log output, try it out!