JavaScript Kata #40: Factorial Factory

By YayoDrayber | Learn To Code With Me | 25 Sep 2022

If you missed the introductory post, it's here. For a list of previously solved katas, please refer to the bottom of this page.

If this is your first time seeing my post, please note - these katas are probably randomly assigned per user so please don't go into Codewars thinking we will have exactly the same user experience, I am just posting these in the order that I got them.

Kata #40

DESCRIPTION:

In mathematics, the factorial of integer 'n' is written as 'n!'. It is equal to the product of n and every integer preceding it. For example: 5! = 1 x 2 x 3 x 4 x 5 = 120

Your mission is simple: write a function that takes an integer 'n' and returns 'n!'.

You are guaranteed an integer argument. For any values outside the positive range, return null, nil or None .

Note: 0! is always equal to 1. Negative values should return null;

For more on Factorials : http://en.wikipedia.org/wiki/Factorial

Starting code:

//This function should return n!
function factorial (n) {
  
}

My attempt that worked:

//This function should return n!
function factorial (n) {
  let result = 1
  if (n > 0) {
    for (i = n ; i > 1  ; i--){
    result *= i 
    } 
    return result
  } else if (n == 0){return 1}
    else {return null}
}

Rank #1 in 'Best Practices':

//This function should return n!
function factorial (n) {
  if (n > -1)
    return n == 0 ? 1 : n * factorial(n - 1);
}

Wow. Gotta admit I got got. Also have to admit I had to look up if there was a built-in factorial function (there wasn't) and realized this was my first taste of recursion challenge in the wild. Although the code I cooked up worked, I'm jealous of those that looks so much cleaner.

Rank #2 also a recursion:

//This function should return n!
function factorial (n) {
  if (n < 0) return null;
  if (n === 0) return 1;
  
  return n * factorial(--n)
}

Rank #3 exactly the same as rank #2 but somehow got detected as different, with the difference only being spaces and semicolons:

//This function should return n!
function factorial (n) {
  if (n < 0) return null
  if (n === 0) return 1
  return n * factorial(n-1)
}

Rank #4 with yet another recursion :

//This function should return n!
function factorial(n) {
  return n>=0 ? n ? n*factorial(n-1) : 1 : null;
}

I mistakenly thought I couldn't use .reduce() because the argument passed to the function is not an array but rank #5 by user andorey demonstrates it here:

// n!
function factorial (n) {
 return n < 0 ? null : Array.from({length: n}, (_,i)=> i+1).reduce((a,b)=> a*b,1) 
}

Rank #6 with a for loop by user g964:

function factorial(n) {
    if (n < 0) return null;
    var val = 1;
    for (var i = 2; i <= n; i++)
        val = val * i;
    return val;
}

Then ranks #7 to #10 also used the recursive solution. Which should probably beg the question, why didn't I think of that? Well because recursion hasn't come up yet in my study material, that's why ¯\_(ツ)_/¯ 🤣

That's it for Kata #40, stay tuned for more katas to be solved!

Link to Kata #1: Square(n) Sum

Link to Kata #2: Convert a Number to a String

Link to Kata #3: DNA to RNA Conversion

Link to Kata #4: Remove First and Last Character

Link to Kata #5: MakeUpperCase

Link to Kata #6: Total amount of points

Link to Kata #7: A Needle in the Haystack

Link to Kata #8: Sum of positive

Link to Kata #9: Basic Mathematical Operations

Link to Kata #10: Beginner - Reduce but Grow

Link to Kata #11: Square Every Digit