Topic 2612 min readStructured Tutorial

JavaScript do-while Loop - Complete Beginner Tutorial

Quick Answer

The JavaScript do-while loop executes a block of code at least once and then repeats it as long as the specified condition remains true. It differs from the while loop by checking the condition after the code block runs, ensuring the code runs at least once.

Learning Objectives

Explain the purpose of do-while Loop in a practical learning context.
Identify the main ideas, terms, and decisions involved in do-while Loop.
Apply do-while Loop in a simple real-world scenario or practice task.

Introduction

Control flow statements allow you to dictate the order in which your code executes. Loops are a fundamental part of control flow in JavaScript.

The do-while loop is a special type of loop that guarantees the code block runs at least once before the condition is tested.

The do-while loop ensures your code runs first, then checks if it should repeat.

Understanding the do-while Loop

The do-while loop executes a block of code once, then continues to execute it repeatedly as long as a specified condition evaluates to true.

This loop is useful when you want the code inside the loop to run at least once regardless of the condition.

Syntax starts with the 'do' keyword followed by a code block.
After the block, the 'while' keyword is used with a condition in parentheses.
The condition is evaluated after the code block executes.
If the condition is true, the loop repeats; if false, the loop ends.

do-while Loop Syntax

Here is the basic syntax of a do-while loop in JavaScript:

do-while Loop Syntax
Keyword	Description
do	Starts the loop and executes the code block once
{ ... }	Code block to execute
while (condition);	Evaluates the condition to decide if the loop repeats

Example of do-while Loop

Let's look at a simple example that prints numbers from 1 to 5 using a do-while loop.

Practical Example

This code initializes count to 1, prints it, then increments count. The loop continues as long as count is less than or equal to 5.

Examples

Print Numbers 1 to 5 Using do-while Loop

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

This code initializes count to 1, prints it, then increments count. The loop continues as long as count is less than or equal to 5.

Best Practices

Use do-while loops when you need the code block to run at least once.
Keep the loop condition simple and clear to avoid infinite loops.
Ensure variables used in the condition are updated inside the loop.
Use meaningful variable names for readability.

Common Mistakes

Forgetting the semicolon after the while(condition) statement.
Not updating the loop variable inside the loop, causing infinite loops.
Using do-while when a while or for loop is more appropriate.
Placing complex logic inside the loop condition, reducing readability.

Hands-on Exercise

Count Down Using do-while Loop

Write a do-while loop that counts down from 10 to 1 and prints each number.

Expected output: 10 9 8 7 6 5 4 3 2 1

Hint: Initialize a variable to 10 and decrement it inside the loop.

Validate User Input

Use a do-while loop to repeatedly prompt the user for a number greater than 100 until they enter a valid number.

Expected output: Loop continues until user enters a number > 100.

Hint: Use prompt() and check the input inside the loop condition.

Interview Questions

What is the main difference between a while loop and a do-while loop in JavaScript?

Interview

A while loop checks the condition before executing the code block, so it may not run at all if the condition is false initially. A do-while loop executes the code block once before checking the condition, ensuring it runs at least once.

When should you use a do-while loop instead of a while loop?

Interview

Use a do-while loop when you want the code inside the loop to execute at least once regardless of the condition, such as when prompting a user for input that needs to be validated.

What is do-while Loop, and why is it useful?

Beginner

The JavaScript do-while loop executes a block of code at least once and then repeats it as long as the specified condition remains true.