JavaScript do…while Loop
Summary: in this tutorial, you will learn how to use the JavaScript do...while statement to create a loop that executes a block until a condition is false.
Introduction to the JavaScript do…while statement
The do...while loop statement creates a loop that executes a block until a condition evaluates to false.
The following statement illustrates the syntax of the do...while loop:
do {
statement;
} while(expression);Code language: JavaScript (javascript)
Unlike the while loop, the do-while loop always executes the statement at least once before evaluating the expression.
Because the do...while loop evaluates expression after each iteration, it’s often referred to as a post-test loop.
Inside the loop body, you need to make changes to some variables to ensure that the expression is false after some iterations. Otherwise, you’ll have an indefinite loop.
Note that starting with ES6+, the trailing semicolon (;) after the while(expression) is optional. So you can use the following syntax:
do {
statement;
} while(expression)Code language: JavaScript (javascript)
The following flowchart illustrates the do-while loop statement:
In practice, you often use the do...while statement when you want to execute the loop body at least once before checking the condition.
JavaScript do while statement examples
Let’s take some examples of using the do…while statement.
1) Simple JavaScript do while statement example
The following example uses the do...while statement to output five numbers from 0 to 4 to the console:
let count = 0;
do {
console.log(count);
count++;
} while (count < 5)Code language: JavaScript (javascript)
Output:
0
1
2
3
4
In this example:
- First, declare and initialize the
countvariable to zero. - Second, show the
countand increase its value by one in each iteration until its value is greater or equal 5.
2) Using the JavaScript do while statement to make a simple number guessing game
The following example uses the do...while statement to generate a number guessing game.
The script generates a random integer between 1 and 10. And you have to make a number of guesses until your number matches the random number.
// generate a secret number between 1 and 10
const MIN = 1;
const MAX = 10;let secretNumber = Math.floor(Math.random() * (MAX - MIN + 1)) + MIN;
let guesses = 0; // for storing the number of guesses
let hint = ''; // for storing hint
let number = 0;
do {
// get input from user
let input = prompt(`Please enter a number between ${MIN} and ${MAX}` + hint);
// get the integer
number = parseInt(input);
// increase the number of guesses
guesses++;
// check input number with the secret number
// provide hint if needed
if (number > secretNumber) {
hint = ', and less than ' + number;
} else if (number < secretNumber) {
hint = ', and greater than ' + number;
} else if (number == secretNumber) {
alert(`Bravo! you're correct after ${guesses} guess(es).`);
}
} while (number != secretNumber);
Code language: JavaScript (javascript)
How it works.
- First, generate a random number between 1 and 10.
- Second, prompt for input an integer and compare it with the secret number. If they are not equal, show a hint. Otherwise, display the congratulation message.
Summary
- Use the
do…whilestatement to create a loop that executes a block until a condition isfalse.