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}.`);
}
```

**Output**

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.