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).

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!

• Uh-oh! We've got trouble... what is that exclamation point doing way down there?

• The first thing to do is DON'T PANIC!

• You are totally going to figure this out.

• And even if you don't, you haven't actually broken anything.

• In fact, it's really hard to break a computer just by typing, so stay calm.

Control-C to close

• First things first: get back to the command line
• This program doesn't exit yet, so you will need to force it to close
• Do this by holding down CONTROL and pressing C
  • abbreviated ⌃C or ^C or CTRL-C

Let's fix this

• Have you figured out what the problem is?
• If not, I'll tell you on the next slide.
• Take a second and try to figure it out first. I'll wait.

The newline character

• Here's a fun fact:
• In addition to letters, numbers, and punctuation, computers also store other keys inside strings
• Among these CONTROL CHARACTERS is the one that represents the RETURN KEY
• This character's name is NEWLINE
• Every time you read a line, the computer reads all the characters, including the newline!

Trim it

• Fortunately, there's an easy fix
• If you send the message trim to a string, it will remove all SPACES and NEWLINES from both ends

LAB: fixing Hello, Friend

• Change the program to look like this:

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

• Run it and make sure it works OK
• Press ⌃C to close it

LAB: This Way To The Exit

• Change the program to look like this:

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

Note that:

• process.exit uses the same process object as process.stdin
• The call to process.exit() must be inside the callback
  • Otherwise, what happens? Try it and see!

LAB: Capitalization

• What happens if you type your name in all lowercase?
• Make the program capitalize your name for you even if you forget.

Hint: remember slice from the Strings lesson?

LAB: YELL YOUR NAME

• Now go crazy and make it YELL your name!
  • Hint: Use the toUpperCase method

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.

CONGRATULATIONS!

You just wrote a program!

You are now officially a coder. HIGH FIVE!

Lab: Name Length

• Change name.js so it also prints the number of characters in the user's name.

• For instance:

  What is your first name? Grace
  What is your last name? Hopper
  Hello, Grace Hopper!
  Your name is 11 characters long.
  

Project: Guess the Number

You may now start on the Guess the Number project.

(You will probably also need to learn about logic and loops to get it working, so don't be afraid to read ahead and to ask for help.)

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