JavaScript Program to Find the Factorial of a Number

In this example, you will learn to write a JavaScript program to calculate the factorial of a number.

To understand this example, you should have the knowledge of the following JavaScript programming topics:

The factorial of a number is the product of all the numbers from 1 to that number. For example,

factorial of 5 is equal to 1 * 2 * 3 * 4 * 5 = 120.

The factorial of a positive number n is given by:

factorial of n (n!) = 1 * 2 * 3 * 4.....n

The factorial of negative numbers do not exist and the factorial of 0 is 1.

Example: Find Factorial

// program to find the factorial of a number

// take input from the user
const number = parseInt(prompt('Enter a positive integer: '));

// checking if number is negative
if (number < 0) {
    console.log('Error! Factorial for negative number does not exist.');

// if number is 0
else if (number === 0) {
    console.log(`The factorial of ${number} is 1.`);

// if number is positive
else {
    let fact = 1;
    for (i = 1; i <= number; i++) {
        fact *= i;
    console.log(`The factorial of ${number} is ${fact}.`);


Enter a positive integer: 5
The factorial of 5 is 120.

In the above program, the user is prompted to enter an integer. Then if...else if...else statement is used to check the condition of a number.

  • When the user enters a negative number, an error message is shown.
  • When the user enters 0, the factorial is 1.
  • When the user enters a positive integer, a for loop is used to iterate over 1 to the number entered by the user to find the factorial.
  • Each number is multiplied and stored in the fact variable.