The while loop

Learn loops that run while a condition holds with while.

The while loop

A loop that runs while a condition is true uses while. The form is while (condition) { ... }. The condition is tested before each iteration; if true the body runs, if false the loop ends.

For example let n = 3; while (n > 0) { console.log(n); n--; } prints 3, 2, 1. Note: without n-- the loop runs forever. In while you write the counter update inside the body yourself; forgetting it leads to an infinite loop.

while vs for

for suits cases where the iteration count is known, while suits cases where the count depends on a condition. In for the init, condition and step are on one line; in while only the condition is there, the rest you write.

For example when reading user input until they type "quit", while is natural: while (input !== "quit") { ... }. For counted cases for is shorter and clearer. Both can do the same job; the choice depends on context.

Exercise Count down from 3

Write a while loop: start with let n = 3;, condition n > 0. In the body first call console.log(n), then n--. The output must be three lines: 3, 2, 1.

Show example solution

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

Test yourself

Match each part to its job

while

Starts the loop
(condition)

Tested each iteration
n--

Decrements counter
{ ... }

Body
What is the output of let n = 2; while (n > 0) { console.log(n); n--; }?
```
let n = 2;
while (n > 0) { console.log(n); n--; }
```
- 1 2
- 2 1 0
- 2 1
- 0 1 2
Show answer

2 1 — n starts at 2, prints down to but not including 0; 2, 1.

Write the while header

let n = 3;
____ (n > 0) {
    console.log(n);
    n--;
}

Show answer

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

Next lesson: for...of

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies