Boolean

On or off, true or false.

In `JavaScript, we can make true or false values in many ways. Let us use some of them here.

let isAdmin = true
let isPoor = false

if (isAdmin) {
  // some secret code to execute here
}

if (isPoor) {
  // oh no, Here is some money
}

Since isAdmin is set to be true, line number 4 is going to pass, and we can run the code for that if statement. But line 8 is not going to pass and the code inside that if statement is not going to run.

let x = 1
let y = 5

let answer = x >= y // if x is equal or bigger than y

if (answer) {
  console.log('Correct answer')
}

Here you can see that variable answer is going to contain the value false since x is not equal or bigger than y. And the if statement is not going to pass.

But if we wanted to make this run if the answer was wrong, then we can use ! to reverse the true / false value.

let x = 1
let y = 5

let answer = x >= y // if x is equal or bigger than y

if (!answer) {
  console.log('Wrong answer')
}

When you work with if statement, you have to be careful not to be confused by “Falsy” values.

if ('') {
  // not going to run
}

if (0) {
  // not going to run
}

if ('') {
  // not going to run
}

And here are some more ways to make true or false values.

let a = !1 // false
let b = !0 // true
let c = ![] // false
let d = !!0 // false
let e = !'' // true
let f = !'' // true
let g = !false // true
let h = !true // false
let i = 'false' // true

But it is important to know what can produce true or false values.

Remember that you are reading the code a lot more than you write it, so it is really important to be clear and keep the code as simple as we can.

undefined is not valid in JSON files while null is ok.