sumOfSquares.js

var square = function(x) {
  return x * x;
}

var sum = function(accumulator, x) {
  return accumulator + x;
} 

function sumOfSquares2(nums) {
  return nums
    .map(square)
    .reduce(sum);
}

console.log(sumOfSquares2([1, 2, 3, 4, 5]));