JavaScript do...while Loop
Syntax, Examples, and Output Walkthrough

Introduction to do...while Loop in JavaScript

The do...while loop is one of JavaScript's control flow statements that ensures the loop body executes at least once — no matter what. If you've ever needed to run a block of code before checking a condition, this loop is your go-to.

Syntax of do...while

do {
  // code to be executed
} while (condition);

How It Works

Unlike a while loop that checks the condition first, the do...while loop executes the block once before checking the condition. It’s a perfect fit for scenarios where an action must occur at least once — like prompting user input, menu systems, or retry logic.

Basic Example

let count = 1;
do {
  console.log("Count is: " + count);
  count++;
} while (count <= 3);

Count is: 1
Count is: 2
Count is: 3

Explanation

Here, the loop starts with count = 1. It prints the value, then increments it. This continues until count exceeds 3. Since the check comes after the execution, even if count was more than 3 initially, the message would still print once.

When the Condition is Initially False

let count = 5;
do {
  console.log("This will run once even though count > 3");
} while (count <= 3);

This will run once even though count > 3

This showcases the beauty — or danger — of do...while. It doesn’t care if the condition fails; the code block will always execute once.

Common Use Case: Validating User Input

let input;
do {
  input = prompt("Enter a number greater than 10:");
} while (parseInt(input) <= 10);
console.log("You entered: " + input);

This code ensures the user enters a number greater than 10 — and keeps prompting until they do. The loop is helpful because the prompt must happen at least once regardless of the input.

Using break and continue with do...while

Breaking the Loop Prematurely

let i = 0;
do {
  if (i === 3) {
    break;
  }
  console.log("i is: " + i);
  i++;
} while (i < 5);

i is: 0
i is: 1
i is: 2

As soon as i hits 3, the loop is broken using break.

Skipping Iterations with continue

let i = 0;
do {
  i++;
  if (i === 2) {
    continue;
  }
  console.log("i is: " + i);
} while (i < 4);

i is: 1
i is: 3
i is: 4

When i equals 2, continue skips that iteration — the print is skipped.

Looping Over Arrays with do...while

const colors = ["red", "green", "blue"];
let index = 0;
do {
  console.log("Color:", colors[index]);
  index++;
} while (index < colors.length);

Color: red
Color: green
Color: blue

This is a basic array iteration. While not the most common method (we usually use for or forEach), it demonstrates that do...while is flexible.

Infinite Loop Warning

let num = 1;
do {
  console.log(num);
  // forgot to increment num
} while (num < 5);

This creates an infinite loop because the condition num < 5 is always true and num never changes. Always ensure your loop will reach a stopping condition!

Summary: When to Use do...while

• When the loop body must run at least once.
• When you're collecting input or performing actions before checks.
• When a check is only relevant after execution (post-condition logic).

Key Takeaways

• do...while executes at least once before checking the condition.
• It's useful for input validation, retry logic, and menu-driven programs.
• Careless use can lead to infinite loops — always control your loop exit.

Challenge Yourself

Try writing a program that keeps asking the user for a correct password until they enter it correctly — use do...while and alert the user each time.