Sum all numbers in a range
I don't know what you would use reduce() for to be honest. But you don't need to loop. We just have a simple arithmetic sequence with a straightforward formula:

$$\sum_{i=min}^{max} i = \frac{(max-min+1)(min+max)}2$$

We just need to pick out which one is larger:

function sumFrom(min, max) {
    return (max-min+1) * (min+max) / 2;
}

function sumAll(arr) {
    return sumFrom(Math.min(...arr), Math.max(...arr));
}


Or we could generalize to any arithmetic sequence:

function sumArithmetic(a1, n, d) {
    return n*(2*a1 + (n-1)*d) / 2;
}

function sumAll(arr) {
    var min = Math.min(...arr);
    return sumArithmetic(min, Math.max(...arr) - min + 1, 1);
}




I guess if you really want reduce() you'd do it this way (not using ES6 because I don't know how to get freecodecamp to use it):

function sumAll(arr) {
  var min = Math.min.apply(null, arr);
  var max = Math.max.apply(null, arr);

  return Array.apply(null, Array(max-min+1))   // get array of correct size
      .map(function(_, b) { return b+min; })   // change it to have correct values
      .reduce(function(a, b) { return a+b; }); // and sum it
}