addWithRecursion.js

const addWithRecursion = x => {
  // base case
  return x <= 0
    // ends function when x hits 0
    ? 0
    // checks if x divides evenly by 2
    : x % 2 === 0
      // retains x and adds it to the function call of x - 1
      ? x + addWithRecursion(x - 1)
      // ignores x and moves on to the function call of x - 1
      : addWithRecursion(x - 1)
}

console.log(addWithRecursion(100)) // 2550