Calculate the factorial of a number using JavaScript

The factorial of a number n is the product of all positive integers less than or equal to n. In mathematics, it is denoted by n!. Calculating it using code can be approached in various ways, such as using recursion or iteration.

Iterative approach

The iterative approach calculates the factorial of a number using a for loop, updating a result variable with the product of all numbers from 2 to n. While it's not the prettiest code you'll write, it gets the job done, it's efficient and easy to understand.

const factorial = n => {
  if (n < 0) throw new TypeError('Negative numbers are not allowed!');
  let result = 1;
  for (let i = 2; i <= n; i++)  result *= i;
  return result;
};

factorial(6); // 720

[!NOTE]

You can easily tweak the code to use Array.prototype.reduce() instead of a for loop, but it's not recommended as it's less efficient.

Recursive approach

The recursive approach is more elegant and concise, but it can be less efficient due to the overhead of function calls. Instead of a loop, it uses a function that calls itself with a smaller input until it reaches the base case of n <= 1, at which point it returns 1.

const factorial = n => {
  if (n < 0) throw new TypeError('Negative numbers are not allowed!');
  return n <= 1 ? 1 : n * factorial(n - 1);
};

factorial(6); // 720