JavaScript while and do…while Loops

Use a while loop when you do not know in advance how many times to repeat. It runs as long as a condition is true.

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let count = 0;

while (count < 3) {
  console.log("count is", count);
  count++;
}

The condition is checked before each round. Something inside the loop must eventually make it false, otherwise it runs forever.

A realistic example

Keep halving a number until it is small.

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let value = 100;
let steps = 0;

while (value > 1) {
  value = value / 2;
  steps++;
}
console.log(steps); // → 7

do…while

A do...while loop checks the condition after each round, so the body always runs at least once.

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let attempts = 0;

do {
  attempts++;
  console.log("Attempt", attempts);
} while (attempts < 3);

Compare with a while whose condition is false from the start:

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let n = 10;

while (n < 5) {
  console.log("while body");   // never runs
}

do {
  console.log("do body");      // runs once
} while (n < 5);

Typical uses

• Reading data until none is left.
• Retrying something until it succeeds (with a limit!).
• Algorithms where the number of steps depends on the data.

Safety: always have a way out

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let tries = 0;
const maxTries = 5;

while (tries < maxTries) {
  tries++;
  if (tries === 3) {
    console.log("Found it on try", tries);
    break;
  }
}

Adding a maximum count guards against accidental infinite loops. Keep the Try it editor away from while (true) without a break: it would freeze the example.