README.md

User Input in NodeJS

Goals

• Understand what user input means in a command line interface (CLI) application
• How to implement require and use modules
• Use readline module to get user input + utilize that input
• Create a CLI application

Keywords

• Modules
• Require
• Threads

Resources

• What is a Thread
• Philip Roberts - JS Event Loop
• Asynchronous vs Synchronous
• What does it mean that Javascript is Asynchronous

Lesson

The Readline Module

NodeJS handles most user input in an asynchronous manner. To illustrate, let's examine the on function that is part of the readline module. A module is reusable code that exists in another file. In order to access a specific module (file), we use the require keyword:

const readline = require('readline')

The readline module is built into node - if you can run node you are able to access readline. The variable to the left of require can have any name, However, The string to the right of require ('readline') must remain exactly the same. This string would be used to find a file whose name is readline. There is one more step to be made to set up a readline interface - this will be given to you as a boilerplate, and it's not something you need to be concerned with at the moment.

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
})

Now we can use the variable rl to handle the user's input. The readline interface asynchronously listens to an input stream coming from the user via keyboard input. It does so on a separate thread, so the execution of JS code does not get interrupted. Readline can respond to certain events - such as a user pressing the enter key. We can provide a callback to readline, and when the user presses enter, readline will put that callback function in the JS task queue, with the user's input as an argument to that function.

To respond to a line of input from the user, we use the readline's on method: we provided a string with the desired event - in this case 'line' and a callback that expects the user's input as arguments.

rl.on('line', function(input) {
  console.log('your input was: ' + input)
})

This isn't terribly useful. With command line applications, you generally give the user options to choose from and then execute a command based on what they type. Think about using git in the terminal--typing git help pulls up a huge menu of options. From there you can type in different commands following git and different commands will be executed.

To do this ourselves, we will implement switch statements, which are good for when you want to do a bunch of else if statements. Let's see:

Boilerplate

These are all the excerpts of code we have used in this lesson to create a node app with readline. Use them as a boilerplate when writing your own node app.

//  importing the readline module
var readline = require('readline')

// We will be able to use `rl`
// Only After creating the readline interface
// creating input, output and prompt methods on the readline interface
var rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  prompt: "Enter a command. Type 'menu' to see commands."
})

// invoke the prompt
rl.prompt()

// Listening to end of line event
// Checks input (here called line) against case statements
rl.on('line', function(line) {
  switch (line.trim()) {
      case 'menu':
        console.log(<GIVE USER MENU CHOICES>);
        break;
      default:
        console.log(`Say what? I might have heard '${line.trim()}'`);
        break;
    }
    rl.prompt();
}).on('close', () => {
  // this is the function that exits the application and closes readline
  console.log('>>Have a great day!');
  process.exit(0);
})