JavaScript Fundamentals · Loops · Lesson 10 of 48

while Loops

Concept

while Loops

A while loop repeats as long as a condition stays true, which is useful when you do not know exactly how many iterations you will need.

Because the condition is checked before every pass, a while loop can also run zero times if the condition is already false the first time it's checked.

By the end of this lesson

Explain the core idea behind while Loops.
Predict output before running a change.
Test one realistic and one unusual input.
Use the result to make the next decision.

How to study this page

1. Read one concept.
2. Change the example.
3. Run, compare, and explain.
4. Complete the challenge below.

Syntax

while (condition) {
  // code to repeat
}

ExampleRunnable

let count = 0;
while (count < 3) {
  console.log("Count:", count);
  count++;
}

Try it Yourself »

do...while: run at least once

let n = 10;
do {
  console.log(n);
  n++;
} while (n < 5);

Note: Even though the condition is false from the start, do...while always runs its body at least once before checking.

Looping until a condition is met

let attempts = 0;
while (attempts < 3) {
  attempts++;
}
console.log("Gave up after", attempts, "attempts");

Self-check before continuing

Without looking at the example, describe what changes when you modify one input in while Loops. Then reopen the editor and prove your explanation with a small test.

You are ready to continue when you can predict, test, and explain the result.

Your turn

Write a while loop that keeps doubling a number starting from 1 until it exceeds 100.

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Console

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