What is and how to use the WHILE and DO-WHILE loop in JavaScript

The while and do-while loops are control structures in JavaScript that allow the repeated execution of a block of code while a specific condition is met.

Unlike the for loop, which is used when the exact number of iterations is known, the while and do-while loops are ideal when the number of iterations is unknown or depends on an exit condition.

while Loop

The while loop repeats a block of code as long as a given condition is true. The basic syntax of a while loop in JavaScript is as follows:

while (condition) {
    // Code to execute while the condition is true
}

The code block is executed repeatedly as long as the specified condition is true. It is important to be careful with the condition to avoid infinite loops.

Basic Example

let counter = 0;

while (counter < 5) {
    console.log(counter);
    counter++;
}

In this example, the while loop will print the numbers from 0 to 4, as the condition counter < 5 evaluates to true during the first five iterations.

do-while Loop

The do-while loop is similar to the while loop, but it guarantees that the code block will execute at least once, even if the condition is false from the start. The basic syntax is as follows:

do {
    // Code to execute at least once
} while (condition);

The code block is executed first, and then the condition is checked. If the condition is true, the block is executed again; if it is false, the loop ends.

Basic Example

let counter = 0;

do {
    console.log(counter);
    counter++;
} while (counter < 5);

This do-while loop will produce the same output as the while loop from the previous example.

Differences between while and do-while

• The while loop checks the condition before each iteration, while the do-while loop checks the condition after each iteration.
• The do-while loop guarantees at least one execution of the code block, while the while loop may not execute the block if the condition is false from the start.

Practical Examples

Counting to 10

In this example, a while loop is used to count from 1 to 10 and print each number.

let counter = 1;

while (counter <= 10) {
    console.log(counter);
    counter++; // Increment the counter by 1
}

This while loop runs while the value of counter is less than or equal to 10. In each iteration, the current value of counter is printed, and then it is incremented by 1.

Summing Positive Numbers Entered by the User

In this example, a while loop is used to sum positive numbers entered by the user until a negative number is entered.

let sum = 0;
let number;

number = parseInt(prompt("Enter a positive number (a negative number to finish): "));

while (number >= 0) {
    sum += number; // Adds the entered number to the 'sum' variable
    number = parseInt(prompt("Enter another positive number (a negative number to finish): "));
}

console.log(`The total sum of positive numbers is: ${sum}`);

This while loop runs as long as the user enters positive numbers. Each number is added to the sum variable. The loop ends when a negative number is entered.

Searching for a Number in a List

In this example, a while loop is used to search for a specific number in a list of numbers.

let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
let searched = 7;
let index = 0;
let found = false;

while (index < numbers.length && !found) {
    if (numbers[index] === searched) {
        found = true; // Marks that the number has been found
        console.log(`Number ${searched} found at position ${index}`);
    }
    index++; // Increment the index
}

if (!found) {
    console.log(`Number ${searched} not found in the list`);
}

This while loop runs as long as the searched number has not been found and the index is within the range of the list.

If the number is found, it is marked as found and its position is printed. If the loop ends without finding the number, it indicates that the number was not found in the list.

User Input Validation

In this example, a do-while loop is used to prompt the user to enter “y” or “n” to confirm if they want to continue.

let response;

do {
    response = prompt("Do you want to continue? (y/n): ").toLowerCase();
} while (response !== "y" && response !== "n");

console.log("Program finished.");

This do-while loop asks the user to enter “y” or “n” to confirm if they want to continue. The code block executes at least once and then repeats while the user does not enter a valid response.