Input and Output

Terminal I/O

process.stdin.on('data', (chunk) => { console.log(chunk) })

node load code, decoded

process.stdin.on('data', (chunk) => { console.log(chunk) })
phrase meaning
process.stdin hey terminal input,
.once('data', ... ) when you get some data,
(chunk) please name it chunk
=> and send it to
{ ... } this block of code
console.log(chunk) print chunk to the terminal

=> is called "fat arrow" and is equivalent to this:

process.stdin.once('data', function(chunk) { console.log(chunk) })

and the block of code itself is called a callback (since you are asking stdin to call you back when it receives input).

LAB: Hello, friend!

  1. Open hello.js in your text editor
  2. Change it to contain the following code:

    console.log("What is your name?");
    process.stdin.once('data', (chunk) => {
        let name = chunk.toString();
        console.log("Hello, " + name + "!");
    });
    
  3. Save the file and switch back to the terminal

  4. Run the program using node hello.js

  5. Type in your name and press the Return key (also called Enter)

What happens? Is this what you expected?

Yikes!

Breathe

Control-C to close

Let's fix this

The newline character

Trim it

LAB: fixing Hello, Friend

        console.log("What is your name?");
        process.stdin.once('data', (chunk) => {
            let name = chunk.toString().trim();
            console.log("Hello, " + name + "!");
        });

LAB: exit

console.log("What is your name?");
process.stdin.once('data', (chunk) => {
    let name = chunk.toString().trim();
    console.log("Hello, " + name + "!");
    process.exit();
});

Note that:

LAB: Capitalization

Hint: remember slice from the Strings lesson?

LAB: YELL YOUR NAME


readline

using readline

To use readline, include the following lines in the top of your source file:

const readline = require('readline');

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

function ask(questionText) {
  return new Promise((resolve, reject) => {
    readlineInterface.question(questionText, resolve);
  });
}

using readline - explanation

code explanation
const readline = require('readline'); load the readline package and name it readline
const readlineInterface = readline.createInterface({...}) create an interface to readline using the following settings:
input: process.stdin, for input, use the standard input stream (i.e. terminal keyboard input)
output: process.stdout for output, use the standard output stream (i.e. terminal console output)
function ask(questionText) {...} create a function named ask that uses the Promise API

(We will cover the promises in much more detail later; for now, all you really need to know is that Promises allow us to use async and await in the next slide.)

LAB: using readline and await

Codealong time! Please follow along with the instructor and enter this code into a file named quest.js:

const readline = require('readline');

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

function ask(questionText) {
  return new Promise((resolve, reject) => {
    readlineInterface.question(questionText, resolve);
  });
}

start();

async function start() {
  let name = await ask('What is your name? ');
  let quest = await ask('What is your quest? ');
  let color = await ask('What is your favorite color? ');
  console.log('Hello ' + name + '! ' +
    'Good luck with ' + quest + ', ' +
    'and here is a ' + color + ' flower for you.');
  process.exit();
}

async and await

1. `await` means "wait for the following thing to happen"
2. when you use `await` inside a function, you must use `async` to define that function

WARNING: async functions don't play nicely with for loops! (Fortunately, there are other ways to loop that do work well.)

LAB: Full Name

CONGRATULATIONS!

You just wrote a program!

You are now officially a coder. HIGH FIVE!

Lab: Name Length

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Outline

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