while and do...while

A while statement creates a loop that executes a specified block of code as long as a test condition evaluates to true.

This is useful when you don’t know exactly how many times you need to repeat an action, but you know the condition that should stop it.

The while loop

The condition in a while loop is evaluated before executing the statements inside the body.

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

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Because the condition is checked first, if the condition is false initially, the loop body is completely skipped:

let condition = false;
while (condition) {
  console.log('This will never run');
}
console.log('Finished');

Finished

Loops are commonly used to accumulate values over time. In this example, the loop adds the current value of n to x until n reaches 3:

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

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The do…while loop

Sometimes you need the loop to run at least once, regardless of whether the condition is true or false. That’s where do...while comes in.

A do...while loop executes a specified statement at least once, and then re-executes it as long as the condition evaluates to true. The condition is evaluated after executing the statement.

let i = 0;
do {
  i += 1;
  console.log(i);
} while (i < 2);

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Notice what happens if the condition is already false when the loop starts. Because the check happens at the end, the body runs once anyway:

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

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Comparing the execution flow

Avoiding infinite loops

Make sure the condition in a loop eventually becomes false—otherwise, the loop will never terminate!

If you do not update the variable that the condition relies on, the program will get stuck running the loop forever, crashing the page or script. Always ensure the loop variable is updated dynamically:

let val = 10;
while (val > 0) {
  val -= 5;
}
console.log(val);

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Common Misconceptions

A very common mistake when learning about while loops is assuming the loop body will always run at least once.

Try to predict the outcome below. Does the loop body execute?

let runs = 0;
while (runs > 0) {
  runs++;
}
console.log(runs);

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Edge Cases

• Omitting curly braces: If you omit the curly braces after a while statement, only the very next statement is considered part of the loop. It is highly recommended to always use curly braces for clarity, as omitting them often accidentally excludes the update statement, causing an infinite loop.
• Intentional infinite loops: Using a truthy value like while (true) intentionally creates an infinite loop that will run forever unless you manually break out of it using a break statement inside the loop body.