isEvenSwitch.js

const isEven = x=> {
  switch (x) {
    // base cases
    case 0: return true;
    case 1: return false
    // subtracts 2 off of x until it hits 0 or 1
    default: return isEven(x - 2)
  }
}

isEven(100) // true
isEven(99) // false