The while loop

Learn the while loop that runs as long as the condition is true.

The while loop

The while loop repeats a block as long as the condition is true. The condition is tested first; if false, the body never runs. That's why while is also called a pre-condition loop.

let count = 3;
while (count > 0) {
  console.log(count);
  count--;
}

Here count decreases each step. When count > 0 becomes false, the loop stops. If you forget count--, the condition stays true forever and the loop never ends; that's an infinite loop.

The infinite loop trap

If the condition never becomes false, the while loop runs forever. This is called an infinite loop and it can freeze the browser.

let n = 1;
while (n > 0) {
  console.log(n);
  // n is never changed
}

Here n stays 1; the condition n > 0 is always true. The loop never ends. To prevent this, the body must change the counter: decrement, increment, or exit with break. Otherwise the program locks up and can only be stopped by closing the browser.

Exercise Count down from 5 to 1

Use a while loop to count down from 5 to 1 and print each number with console.log. Decrement the counter each step.

Show example solution

let count = 5;
while (count > 0) {
  console.log(count);
  count--;
}

Test yourself

When is the condition checked in a while loop?
- After the loop
- Only once at the start
- Before each step
- Only on errors
Show answer

Before each step — while tests the condition before each step. If the first test is false, the body never runs.

Decrement the counter to avoid an infinite loop

let count = 3;
while (count > 0) {
  console.log(count);
  ____;
}

Show answer

let count = 3;
while (count > 0) {
  console.log(count);
  count--;
}

What does this code print?
```
let count = 3;
while (count > 0) {
  console.log(count);
  count--;
}
```
- 3, 2, 1
- 1, 2, 3
- 3, 2, 1, 0
- 0, 1, 2, 3
Show answer

3, 2, 1 — count starts at 3, is printed while greater than 0, and decreases. So 3, 2, 1 are printed.

Next lesson: The do...while loop

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