Functional solution to Free Code Camp's "Sum All Numbers in a Range" challenge

sumAll.js

function sumAll(x) {
  return Array.from(
    {length: Math.max(...x) - Math.min(...x) + 1},
    (v,k) => k + Math.min(...x)
  ).reduce((s, c) => s + c)
}

1. function header expected by Free Code Camp's tests, x is an array
2. create a new array to hold our range (docs), note: we return the final summed result

3. elements will be (max - min + 1), the +1 is needed to include the upper value, e.g. 5..10 = (10 - 5 + 1) = (5 + 1) = 6

4. lambda populates array with incrementing values starting from min (the array keys k already increment)
5. reduce array by summing current number c into sum s
6. Note: min() and max() expect several numbers as arguments, not a single array, ...x is how we spread x into arguments