JavaScript while and do...while Loop

JavaScript while Loop

The while loop repeatedly executes a block of code as long as a specified condition is true.

The syntax of the while loop is:

while (condition) {
    // body of loop
}

Here,

1. The while loop first evaluates the condition inside ( ).
2. If the condition evaluates to true, the code inside { } is executed.
3. Then, the condition is evaluated again.
4. This process continues as long as the condition evaluates to true.
5. If the condition evaluates to false, the loop stops.

Flowchart of while Loop

Example 1: Display Numbers From 1 to 3

// initialize variable i
let i = 1;

// loop runs until i is less than 4
while (i < 4) {
    console.log(i);
    i += 1;
}

Output

1
2
3

Here is how the above program works in each iteration of the loop:

To learn more about loop conditions, visit JavaScript Comparison and Logical Operators.

Example 2: Sum of Only Positive Numbers

let num = 0, sum = 0;

// loop as long as num is 0 or positive
while (num >= 0) {

    // add all positive numbers
    sum += num;

    // take input from the user
    num = parseInt(prompt("Enter a number: "));
}

// last, display sum
console.log(`The sum is ${sum}`);

Output

Enter a number: 2
Enter a number: 4
Enter a number: -3
The sum is 6

The above program prompts the user to enter a number.

Since JavaScript prompt() only takes inputs as string, parseInt() converts the input to a number.

As long as we enter positive numbers, the while loop adds them up and prompts us to enter more numbers.

So when we enter a negative number, the loop terminates.

Finally, we display the total sum of positive numbers.

Note: When we add two or more numeric strings, JavaScript treats them as strings. For example, "2" + "3" = "23". So, we should always convert numeric strings to numbers to avoid unexpected behaviors.

JavaScript do...while Loop

The do...while loop executes a block of code once, then repeatedly executes it as long as the specified condition is true.

The syntax of the do...while loop is:

do {
    // body of loop
} while(condition);

Here,

1. The do…while loop executes the code inside { }.
2. Then, it evaluates the condition inside ( ).
3. If the condition evaluates to true, the code inside { } is executed again.
4. This process continues as long as the condition evaluates to true.
5. If the condition evaluates to false, the loop terminates.

Flowchart of do...while Loop

Example 3: Display Numbers from 3 to 1

let i = 3;

// do...while loop
do {
    console.log(i);
    i--;
} while (i > 0);

Output

3
2
1

Here, the initial value of i is 3. Then, we used a do...while loop to iterate over the values of i. Here is how the loop works in each iteration:

let i = 0;

// false condition
// body executes once
do {
    console.log(i);
} while (i > 1);

// Output: 0

let i = 0;

// false condition
// body not executed
while (i > 1) {
    console.log(i);
};

Example 4: Sum of Positive Numbers

let sum = 0, num = 0;

do {

    // add all positive numbers
    sum += num;

    // take input from the user
    num = parseInt(prompt("Enter a number: "));

    // loop terminates if num is negative
} while (num >= 0);

// last, display sum
console.log(`The sum is ${sum}`);

Output

Enter a number: 2
Enter a number: 4
Enter a number: -3
The sum is 6

In the above program, the do...while loop prompts the user to enter a number.

As long as we enter positive numbers, the loop adds them up and prompts us to enter more numbers.

If we enter a negative number, the loop terminates without adding the negative number.

let i = 1;

// always true condition
while(i < 5) {
    console.log(i);
}

let i = 5;

// always true condition
do {
    console.log(i);
} while (i > 1);

// display hi 3 times

for (let i = 1; i <= 3; i++) {
    console.log("hi");
}

console.log("bye");

hi
hi
Hi
bye

// display hi as long as user wants
let isDisplay = true;
let userChoice = "";

while (isDisplay) {
    console.log("hi");

    userChoice = prompt("print hi again? y for yes: ");
    if (userChoice != "y")
        isDisplay = false;
}

console.log("bye");

hi
print hi again? y for yes: y
hi
print hi again? y for yes: y
hi
print hi again? y for yes: n
bye

Also Read:

• JavaScript break Statement
• JavaScript continue Statement