Truthiness

Computers have a very particular idea of when things are true and false.

True or False?

Try the following in node:

  • 1 < 2
  • 2 + 2 < 4
  • 2 + 2 <= 4
  • "anonymous".endsWith("us")
  • "every journey".startsWith("a step")

Comparisons

Comparison operators let you compare two values. JavaScript has all the usual suspects...

Operator Comparison
< less than
> greater than
<= less than or equal to
>= greater than or equal to
== equal to
!= not equal
=== really equal to
!== really not equal to

MDN: comparison operators

These are also called "Boolean operators" after George Boole, a 19th-century mathematician who invented Boolean algebra.)

Conditions

The magic word if is a conditional.

The phrase immediately after if is a condition.

if (age < 18) {
  console.log('Sorry, you cannot vote yet.');
}
phrase meaning
if ( ... ) if this condition's value is truthy
{ ... } then run this block of code

Wait a second. "Truthy?"

MDN: if...else

What is truthiness?

Truthiness

  • in the Colbert Report, truthiness means things we feel to be true, even though we know they're probably not

  • In JavaScript, all values have truthiness unless they are defined as falsy.

  • MDN: Truthy

What is falsiness?

false, null, undefined, 0, NaN, and the empty string ("") are all falsy.

Fortunately, true is truthy and false is falsy.

Unfortunately, the string "false" is truthy, and the string "0" is truthy, even though the number 0 is falsy. This is because the string contains a character, and, even though the character is 0, any string with at least one character is truthy.

MDN: Falsy

if... then... else...

The magic word else allows BRANCHING.

if (age >= 18) {
  console.log('allowed');
} else {
  console.log('denied');
}

Like a fork in the road, the program chooses one path or the other.

It takes the first path if the condition is truthy, and takes the second path if the condition is falsy.

2 + 2 = ?

Sadly, this mathematical expression:

2 + 2 = 4

causes an error. You need to do

2 + 2 == 4

instead. Why?

Condition or Assignment?

BEWARE of using a single equal sign inside an if condition!

  • the value of a comparison is either true or false

    • so if (x == 2) means if x is 2 which changes based on x
  • the value of an assignment is the value being assigned

    • so if (x = 2) means if 2 which is always truthy
    • also, the value of x will be 2 afterwards, no matter what it was before

Different Kinds of Equals Signs

In addition to = and ==, JavaScript also has ===.

That's three equal signs in a row.

Operator Operation Example Meaning
= assignment X = Y let X equal Y
== comparison (fuzzy) X == Y does X mostly equal Y?
=== comparison (exact) X === Y does X really equal Y?

== means "does X equal Y, or if not, can Y be converted into something that equals X?"

Since the rules for type conversion are confusing, most JavaScript experts recommend:

always use ===, never use ==

Using == can have some very interesting side effects, see Stackoverflow

Logical Conjunctions

You can make more complicated logical expressions using conjunctions:

Conjunction Operator Example Meaning
AND && X && Y "are both X and Y true?"
OR || X || Y "is either X or Y (or both) true?"
NOT ! !X "is X false?"

For example:

if (age >= 18 || hasPermissionSlip()) {
  console.log('allowed');
} else {
  console.log('denied');
}

MDN: logical operators

LAB: Good Friend, Bad Friend

In this lab you will create a program to read input from a human user, and then make a decision about what to output as a message based on that input.

Start with a hello.js program that looks like this:

console.log('What is your name?');

function handleInput(chunk) {
  let name = chunk.toString().trim();
  console.log('Hello, ' + name + '!');
}

process.stdin.once('data', handleInput);
  • 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!"

Good Friend, Bad Friend solution

Solution

console.log('What is your name?');

function handleInput(chunk) {
  let name = chunk.toString().trim();
  if (name === 'Darth') {
    console.log('Noooooo! That is impossible!')
  } else {
    console.log('Hello, ' + name + '!');
  }
}

process.stdin.on('data', handleInput);

Lab: 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

Infinite Names solution

Hint 1
Using `.on` instead of `.once` will keep the process running
Hint 2
Remember

process.exit()

will end your program.

Solution

console.log('What is your name?');

function handleInput(chunk) { let name = chunk.toString().trim(); if (name === 'bye!') { process.exit() } else if (name === 'Darth') { console.log('Noooooo! That is impossible!\nWhat is your name?') } else { console.log('Hello, ' + name + '!\nWhat is your name?'); } }

process.stdin.on('data', handleInput);

LAB: 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, Darth Vader, Voldemort, Palpatine, Lex Luthor...
  • Bonus Challenge: don't let enemies sneak in even if they spell their names with capital letters, like VolDeMort

Enemies List solution

Hint
You can do multiple checks in a single if statement by using the logical "or" operator: `||`

if(checkOne || checkTwo || checkThree) {
  //this block of code runs if any of the checks are true
}

Solution

console.log('What is your name?');

function handleInput(chunk) => {
  let name = chunk.toString().trim();
  if (name === "bye!") {
    process.exit();
  } else if (name === 'Darth' || name === 'Sauron' || name === 'Voldemort') {
    console.log('Noooooo! That is impossible!\nWhat is your name?')
  } else {
    console.log('Hello, ' + name + '!\nWhat is your name?');
  }
}

process.stdin.on('data',);