JavaScript While

Repeat while a test is true

A while loop checks a test, runs a block, then checks again. As long as the test is true, the block repeats. When the test is false, JavaScript leaves the loop and continues with the next statement.

Something inside the loop must change the values the test looks at. If nothing changes, the test never becomes false and the script does not stop.

A complete while loop

This loop prints 1 through 5. n starts at 1. Each pass records the current value, then adds 1. When n becomes 6, the test is false and the loop ends.

let n = 1;
const lines = [];

while (n <= 5) {
  console.log(n);
  lines.push(String(n));
  n = n + 1;
}

document.body.textContent = lines.join(", ");

The three parts you manage

1. Set a starting value before the loop.
2. Write a test that can become false.
3. Update that value inside the loop.

Forget step 3 and the test stays true forever. Forget step 1 and the test uses undefined. Thefor loop packs these three into one line. while leaves them visible, which is useful when the update is not a simple plus one — waiting for a flag, shrinking a number, or draining a queue.

Drain a list

while (queue.length > 0) is a natural shape: the test is “is there still work?”shift removes the front item. Length falls. Eventually the test fails.

const queue = ["draft", "review", "ship"];
const done = [];

while (queue.length > 0) {
  const item = queue.shift();
  console.log(item);
  done.push(item);
}

document.body.textContent = "done: " + done.join(", ");

do...while runs the body first

A do...while loop runs the block once, then checks the test. If the test is true, it runs again. The body always executes at least once, even when the test starts false.

let n = 8;
const lines = [];

do {
  console.log(n);
  lines.push(String(n));
  n = n + 1;
} while (n <= 5);

document.body.textContent = "ran once: " + lines.join(", ");

n starts at 8, which already fails n <= 5. A plain while would skip the body. do...while still records 8, then checks and stops.

while versus for

What to remember

• Initialize before the loop. Update inside it. The test must be able to fail.
• while checks first. do...while runs once, then checks.
• A shrinking list or a rising counter are the usual ways to stop.
• Next chapter: break leaves now; continue skips to the next pass.

Next: break and continue — leave a loop early, or skip the rest of this round.