JavaScript while and do...while Loops

While a for loop is ideal when you know exactly how many times to repeat something, the while and do...while loops are built for the opposite case: repeating code until some condition becomes false, without necessarily knowing in advance how many iterations that will take.

The while Loop

A while loop checks its condition before each iteration. If the condition is false from the very start, the loop body never runs at all.

let count = 1;

while (count <= 5) {
  console.log(count);
  count++;
}

Output:

1
2
3
4
5

Notice the counter variable has to be declared and incremented manually, unlike a for loop where that's built into the syntax. Forgetting to update the counter is the most common cause of an accidental infinite loop.

The do...while Loop

A do...while loop is nearly identical, but it checks its condition after running the body — which guarantees the loop body executes at least once, even if the condition is false immediately.

let attempts = 0;
const maxAttempts = 3;

do {
  attempts++;
  console.log(`Attempt ${attempts}`);
} while (attempts < maxAttempts);

Output:

Attempt 1
Attempt 2
Attempt 3

This "run at least once" guarantee is useful for things like menu prompts or retry logic, where you always want to attempt the action at least a single time before checking whether to continue.

while vs do...while: The Key Difference

let x = 10;

while (x < 5) {
  console.log("while: this never runs");
}

do {
  console.log("do...while: this runs once, even though the condition is false");
} while (x < 5);

Output:

do...while: this runs once, even though the condition is false

In practice, plain while loops are far more common than do...while — reach for do...while specifically when the "run it first, then decide whether to repeat" behavior matches your use case.

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