Input and Output


Memory vs I/O


Terminal I/O

The weirdness is explained on the next slide!


node load code, decoded

function handleInput(input) {
  let inputAsString = input.toString();
  console.log("The input is: ", inputAsString);
}

process.stdin.once('data', handleInput)

console.log("Waiting for input...");

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

phrase meaning
process.stdin hey terminal input,
.once('data', ...) when you get some data,
function handleInput(input) please name it input
{ ... } send it to this block of code
input.toString() convert it to a string
console.log(inputAsString) and print it to the terminal

Welcome to Callback City!

The previous code is equivalent to this:

process.stdin.once('data', function handleInput(input) { 
  let inputAsString = input.toString();
  console.log(inputAsString);
});

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


Readline


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

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:
process.stdin, for input, use the standard input stream (i.e. terminal keyboard input)
process.stdout for output, use the standard output stream (i.e. terminal console output)
function ask(questionText) {...} a function named ask that uses the Promise API to asynchronously ask a question and wait for a reply

(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();
}

Async 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


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);
  });
}

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()

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