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 + "!");
});

1. Save the file and switch back to the terminal
2. Run the program using node hello.js
3. 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 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

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

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!

Good Friend, Bad Friend

• Your hello.js program should currently look something like this:

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

• Now change hello.js so that it doesn't always say hello!
  • If the user's name is "Darth" then say "Noooooo! That's impossible!"

Infinite Names

• Change hello.js so it keeps asking for names forever...
  • ...unless and until someone says their name is "bye!"
  • then it stops and exits back to the terminal

Enemies List

• Change hello.js so that it says "Go away!" if the user's name is any one of a number of evil names
• For instance, Voldemort, Satan, Lex Luthor...
• Bonus: don't let enemies sneak in even if they spell their names with capital letters, like VolDeMort

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