JavaScript do...while Loop

What You'll Learn

In this guide, you'll learn how the do...while loop works in JavaScript, understand its syntax, and use it in various examples.

Understanding the do...while Loop

The do...while loop is similar to the while loop, but with one important difference: it always executes the code block at least once, even if the condition is false.

Here's the basic syntax:

do {
    // Code block to execute
} while (condition);

do {
    // Code block to execute
} while (condition);

• Code block: This block runs once before the condition is checked. It will continue to run as long as the condition is true.
• Condition: This is the expression evaluated after each iteration. If it returns true, the loop runs again.

Just like with while loops, be cautious with infinite loops. If the condition always remains true, the loop will never stop.

Example 1: Simple do...while Loop

In this example, we will print the numbers 1 to 5 using a do...while loop in JavaScript.

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

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

How It Works:

• let i = 1: This initializes the variable i to 1.
• do { ... } while (i <= 5): The loop block runs once first, then checks the condition. It repeats while i is less than or equal to 5.
• console.log(i): Prints the current value of i to the console.
• i++: Increments i by 1 after each iteration.

Output

1
2
3
4
5

1
2
3
4
5

Example 2: Print Even Numbers from 1 to 20

In this example, we will print the even numbers from 1 to 20 using a do...while loop in JavaScript.

let i = 2;
do {
    console.log(i);
    i += 2;
} while (i <= 20);

let i = 2;
do {
    console.log(i);
    i += 2;
} while (i <= 20);

How It Works:

• let i = 2: Initializes the counter variable to 2, the first even number.
• do { ... } while (i <= 20): Executes the code block once, then checks the condition.
• console.log(i): Prints the current even number.
• i += 2: Increments i by 2 to move to the next even number.
• Make sure the increment matches the desired pattern. In this case, +2 ensures only even numbers are printed.

Output

2
4
6
8
10
12
14
16
18
20

2
4
6
8
10
12
14
16
18
20

Example 3: Sum of Digits

In this example, we’ll calculate the sum of the digits of a number (e.g., 1234) using a do...while loop in JavaScript.

let num = 1234;
let sum = 0;

do {
    let digit = num % 10; // Get the last digit
    sum += digit;         // Add the digit to the sum
    num = Math.floor(num / 10); // Remove the last digit
} while (num > 0);

console.log("Sum of digits is:", sum);

let num = 1234;
let sum = 0;

do {
    let digit = num % 10; // Get the last digit
    sum += digit;         // Add the digit to the sum
    num = Math.floor(num / 10); // Remove the last digit
} while (num > 0);

console.log("Sum of digits is:", sum);

How It Works:

• let num = 1234: Initializes the number whose digits we want to add.
• let sum = 0: Stores the running total of the digit sum.
• do { ... } while (num > 0): Runs the code block at least once to process digits.
• num % 10: Gets the last digit of num.
• sum += digit: Adds the digit to the total sum.
• Math.floor(num / 10): Removes the last digit from num.
• Important: The loop runs until num becomes 0. If not updated, it may create an infinite loop.

Output

Sum of digits is: 10

Sum of digits is: 10

Example 4: Infinite Loop

In this example, we create an infinite loop using a do...while loop in JavaScript. Be careful—this loop will keep running forever unless stopped manually.

do {
    console.log("This is an infinite loop!");
    // No condition or break to stop the loop
} while (true);

do {
    console.log("This is an infinite loop!");
    // No condition or break to stop the loop
} while (true);

How It Works:

• do { ... } while (true): The condition is always true, so the loop never stops on its own.
• console.log(...): Prints the message repeatedly to the console.
• Important: This loop will continue to run indefinitely, which can freeze your browser or application. To stop it, follow the instructions based on your operating system:
• Windows: Press Ctrl + C in the terminal or command prompt to stop the loop.
• Mac: Press Command + C in the terminal to stop the loop.
• Linux: Press Ctrl + C in the terminal to stop the loop.

Output

This is an infinite loop!
This is an infinite loop!
... (and it keeps printing indefinitely)

This is an infinite loop!
This is an infinite loop!
... (and it keeps printing indefinitely)

Exercises

let i = 1;
do {
    console.log(i);
    i++;
} while (i <= 10);

let i = 1;
do {
    console.log(i);
    i++;
} while (i <= 10);

let i = 1;
let sum = 0;

do {
    sum += i;
    i++;
} while (i <= 5);

console.log("Sum is:", sum);

let i = 1;
let sum = 0;

do {
    sum += i;
    i++;
} while (i <= 5);

console.log("Sum is:", sum);

let response;

do {
    response = prompt("Type 'yes' to continue:");
} while (response !== "yes");

console.log("Thanks!");

let response;

do {
    response = prompt("Type 'yes' to continue:");
} while (response !== "yes");

console.log("Thanks!");

*Try changing conditions and values to explore different outcomes.

What's Next?

Coming up next, we’ll explore for loops — a powerful and commonly used control structure for iterating over sequences like arrays or performing a block of code a specific number of times. Mastering for loops is key to writing clear and efficient repetitive logic in your programs.