The while Loop in JavaScript

The simplest loop is the while loop. It keeps repeating a block of code while a condition is true:

JavaScript

let countdown = 5;

while (countdown > 0) {
  console.log(countdown);
  countdown--;
}

console.log("🚀 LIFTOFF!");

It prints

5
4
3
2
1
🚀 LIFTOFF!

It looks a lot like an if statement, doesn't it? The difference is that an if runs its block once, but a while keeps going back to the top and checking the condition again, over and over, until it becomes false.

Let's trace through it:

Round	`countdown`	`countdown > 0`?	What happens
1	5	true	print 5, countdown becomes 4
2	4	true	print 4, countdown becomes 3
3	3	true	print 3, countdown becomes 2
4	2	true	print 2, countdown becomes 1
5	1	true	print 1, countdown becomes 0
6	0	false	stop looping!

Each trip through the loop is called an iteration. Our countdown loop iterated five times.

Every good loop has three ingredients:

A starting point: let countdown = 5;
A condition that decides whether to keep going: countdown > 0
An update that moves things toward the end: countdown--

Forget any one of them, and things go wrong. Especially the third one...

Reading is step one