How to find factorial of a number in javascript

Finding a factorial is one of the most popular tasks for demonstrating how recursion works, regardless of programming language. And JavaScript isn’t an exception. Also, it’s one of the easiest algorithms, that might be asked on the interview for an entry position. So, finding factorial of a number is your first step on the way to mastering a recursion. Let’s get started.

Standard solution

Of course, you can implement it without recursion, I’ll show you a solution with loop for comparing:

const factorial = (num) => {
  let value = 1
  for (let i = 2; i <= num; i++) {
    value = value * i
  }
  return value
}

factorial(5) // 120

Great. Let’s go to the recursion version.

Single-line solution

For use the second way the factorial function has to call itself with a decremented argument while the last goes to the 1.

const factorial = x => (x === 1) ? 1 : x * factorial(x - 1)

Example of use:

factorial(5) // 120

Explanation

As you can see our factorial function calls itself with decremented number value, it’ll create a call stack of function calls that will close when the function calls with a 1 and returns a 1 as a result.

In other words, the call stack looks like:

f(5) =
  5 * f(4) =
  5 * (4 * f(3)) =
  5 * (4 * (3 * f(2))) =
  5 * (4 * (3 * (2 * f(1)))) =
  5 * (4 * (3 * (2 * (1)))) =
  5 * (4 * (3 * (2))) =
  5 * (4 * (6)) =
  5 * (24) =
  120

Conclusions

So, we’ve written a function for calculation recursion in javascript. Also, we’ve considered how the call stack works with the recursion.

Further Learning:

Tail call optimization in javascript