3.2 Recursion

iseven.js

/*
We’ve seen that % (the remainder operator) can be used to test whether a number is even or odd by using % 2 to check whether it’s divisible by two. Here’s another way to define whether a positive whole number is even or odd:

Zero is even.

One is odd.

For any other number N, its evenness is the same as N - 2.

Define a recursive function isEven corresponding to this description. The function should accept a number parameter and return a Boolean.

Test it on 50 and 75. See how it behaves on -1. Why? Can you think of a way to fix this?
*/

var isEven = function( number ) {
  if( number === 0 ) // if 0
    return true;
  else if( number === 1 ) // if 1
    return false;
  else if( number < 0 ) // if negative number
    return isEven( number * -1 ); // multiply by -1 to make it positive
  else
    return isEven( number - 2 ); // recursion
}

console.log(isEven(50));
// → true
console.log(isEven(75));
// → false
console.log(isEven(-1));
// → ??