JavaScript · Chapter 33 of 55

JavaScript While Loop

A `while` loop repeats a block of code as long as a condition remains true, checking the condition before each iteration. It is ideal when the number of repetitions isn't known in advance.

A `do...while` loop is similar but always executes the body at least once, since it checks the condition after running the block.

while loop

`while (condition) { ... }` checks the condition first; if false immediately, the body never runs.

do...while loop

`do { ... } while (condition);` guarantees the body runs at least once before checking the condition.

Example 1 (javascript)

let i = 0;
while (i < 3) {
  console.log(i);
  i++;
}

Output

0
1
2

The loop checks i < 3 before every iteration.

Example 2 (javascript)

let i = 5;
do {
  console.log(i);
  i++;
} while (i < 3);

Output

do...while runs once even though the condition is false immediately.

Key points

while checks its condition before running the loop body.
do...while always runs at least once.
Both loops need a way to eventually make the condition false.
Use while when the number of iterations is unknown ahead of time.

💡 Note: Forgetting to update the loop's controlling variable is the most common cause of infinite while loops.

JavaScript While Loop

while loop

do...while loop

Key points

📝 Quick Quiz