Input and Output

• Computers have many senses -- keyboard, mouse, network card, camera, joystick, etc. Collectively, these are called INPUT.

• Computers can also express themselves in many ways -- text, graphics, sound, networking, printing, etc. Collectively, these are called OUTPUT.

• Input and Output together are called I/O.

• the only part of your laptop that is really a computer is the CPU and the RAM; all the other parts (keyboard, trackpad, display, disk drive, etc.) are technically I/O devices

Memory vs I/O

• Performing calculations and accessing memory is very fast
• ...but reading and writing to I/O devices is slow
  • (at least as far as the CPU is concerned)
  • I/O operations can take seconds or milliseconds; CPU operations take nanoseconds
• Every time you ask JavaScript to do an I/O operation, it pauses your program
  • this allows the CPU to spend time doing other things, not just sitting idle waiting for a key to be pressed or a file to be written
• In NodeJS, you have to write a function for JavaScript to run once it resumes
  • this function is named an asynchronous callback
  • asynchronous is Greek for "out of time" or "not together in time"

Terminal I/O

• In JavaScript,

  • console.log means "print a line to the terminal"

• In NodeJS,

  • process.stdin means "input coming from the terminal"
  • Reading a line in NodeJS is weird; here's one way to do it

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

The weirdness is explained on the next slide!

node load code, decoded

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

once is a function that takes two parameters, and its second parameter is another function

Welcome to Callback City!

The previous one-liner code is equivalent to this:

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

The printLine function itself is called a callback (since you are asking the I/O device to call you back when it receives input).

readline

• NodeJS is more than a JavaScript interpreter
• It's also a collection of JavaScript libraries
• One of the libraries is called readline
  • readline makes it easier to read lines, naturally :-)
  • the "books" in this library are functions
    • (and classes and other things too)

using readline

Warning: this code uses features we have not yet covered! Copy and paste it verbatim during the codealong below, and don't worry if it doesn't make much sense yet.

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

const readline = require('readline');
const readlineInterface = readline.createInterface(process.stdin, process.stdout);

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

This is called "boilerplate code" -- you don't need to fully understand it before using it.

using readline - explanation

(We will cover the Promise API 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(process.stdin, 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();
}

• run it from the command line using node quest.js

async and await

• We will learn a lot more about callbacks, promises, and async/await later
• For now, follow these two rules when using 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

• Now it's your turn to write a program from scratch.
• Write a program named name.js that asks two things:
  1. Your first name
  2. Your last name

• Then it says hello to the user by their full name.

• Run the program by typing node name.js on the command line.

Full Name solution

const readline = require('readline');
const readlineInterface = readline.createInterface(process.stdin, process.stdout);

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

const readline = require('readline');
const readlineInterface = readline.createInterface(process.stdin, process.stdout);

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

async function fullName() {
  let firstName = await ask("What is your first name? ")
  let lastName = await ask("What is your last name? ")

  console.log("Hello, " + firstName + " " + lastName + "!")
}

fullName()