How to Use While Loop in JavaScript: Syntax and Examples

In JavaScript, a while loop repeatedly runs a block of code as long as the given condition is true. It checks the condition before each loop iteration, so if the condition is false initially, the code inside the loop won't run.

Syntax

The while loop syntax consists of the keyword while, followed by a condition in parentheses, and a block of code inside curly braces. The loop runs the code block repeatedly as long as the condition is true.

• while: starts the loop
• (condition): a test that must be true to continue looping
• { ... }: the code to repeat

javascript

while (condition) {
  // code to run repeatedly
}

Example

This example shows a while loop that counts from 1 to 5 and prints each number. The loop stops when the number becomes greater than 5.

javascript

let count = 1;
while (count <= 5) {
  console.log(count);
  count++;
}

Output

1
2
3
4
5

Common Pitfalls

One common mistake is forgetting to update the condition variable inside the loop, which causes an infinite loop. Another is using a condition that is always false, so the loop never runs.

Here is an example of an infinite loop and its fix:

javascript

// Wrong: infinite loop
let i = 0;
while (i < 3) {
  console.log(i);
  // missing i++ update
}

// Correct: update i to avoid infinite loop
let j = 0;
while (j < 3) {
  console.log(j);
  j++;
}

Output

0
1
2

Quick Reference

Remember these tips when using while loops:

• Always ensure the condition will become false eventually to stop the loop.
• Update variables inside the loop to avoid infinite loops.
• while loops check the condition before running the code block.
• Use break to exit the loop early if needed.

Key Takeaways

A while loop runs code repeatedly as long as its condition is true.

Always update the condition variable inside the loop to avoid infinite loops.

The condition is checked before each loop iteration, so the loop may not run at all if false initially.

Use break to exit a while loop early if needed.

While loops are useful when you don't know in advance how many times you need to repeat.