## 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 can't 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? • 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 = ?

``````2 + 2 = 4
``````

causes an error. You need to do

``````2 + 2 == 4
``````

## The Tragedy of the Equal Sign

• a single equal sign means ASSIGNMENT
• `name = "Alice"` -- "assign the value 'Alice' to the variable 'name'"
• two equal signs means COMPARISON
• `name == "Alice"` -- "does the variable 'name' contain the string 'Alice'?"

This is confusing! (More about it on the next slide.)

"A notorious example for a bad idea was the choice of the equal sign to denote assignment. It goes back to Fortran in 1957 and has blindly been copied by armies of language designers. Why is it a bad idea? Because it overthrows a century old tradition to let "=" denote a comparison for equality, a predicate which is either true or false. But Fortran made it to mean assignment, the enforcing of equality... `x = y` does not mean the same thing as `y = x`."

Niklaus Wirth, Good Ideas, Through the Looking Glass (2005)

## 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

## The Tragedy of the Threequal Sign

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

## Conjunction Junction

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

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

## Good Friend, Bad Friend solution

Solution
``````console.log("What is your name?");
process.stdin.on('data', (chunk) => {
let name = chunk.toString().trim();
if (name === "Darth") {
console.log("Noooooo! That's impossible!")
} else {
console.log("Hello, " + name + "!");
}
});
``````

## 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()
``````

Solution
``````console.log("What is your name?");
process.stdin.on('data', (chunk) => {
let name = chunk.toString().trim();
if (name === "bye!") {
process.exit()
}else if (name === "Darth") {
console.log("Noooooo! That's impossible!\nWhat is your name?")
} else {
console.log("Hello, " + name + "!\nWhat is your name?");
}
});
``````

## 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, Voldemort, Satan, Lex Luthor...
• Bonus: 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?");
process.stdin.on('data', (chunk) => {
let name = chunk.toString().trim();
if (name === "bye!") {
process.exit()
}else if (name === "Darth" || name === "Sauron" || name === "Voldemort") {
console.log("Noooooo! That's impossible!\nWhat is your name?")
} else {
console.log("Hello, " + name + "!\nWhat is your name?");
}
});
``````