The factorial of a number n (written as n!) is the product of all positive integers from 1 to n.

Example:

5! = 5 × 4 × 3 × 2 × 1 = 120

The general formula is:

n! = n × (n-1) × (n-2) × ... × 1

By definition, 0! = 1.

Simple Factorial Function 

function factorial(n) {
  if (n === 0 || n === 1) return 1;
  return n * factorial(n - 1);
}

Example: Calculate factorial(4)

Step-by-step breakdown:

factorial(4)
= 4 × factorial(3)
= 4 × (3 × factorial(2))
= 4 × (3 × (2 × factorial(1)))
= 4 × (3 × (2 × 1))
= 4 × (3 × 2)
= 4 × 6
= 24

On code: 

console.log(factorial(4)); // Output: 24