Sum all numbers in a range

I have implemented the "Sum All Numbers in a Range" challenge from Free Code Camp quoted below.

The code works like a charm, but I don't think it's idiomatic. The challenge hints imply I should be using Math.max(), Math.min() and Array.reduce(). In addition I used the spread operator from ES6 to simplify both Math calls.

I'm looking for a more idiomatic way to solve this. I think it can be done in a one-liner, something like:

return arr.sort((function(a, b) { return a - b; })).reduce(function(a, b) { /* */ });

But I'm pretty sure I'm heading the wrong way with the above.

The challenge:

We'll pass you an array of two numbers. Return the sum of those two numbers and all numbers between them.

The lowest number will not always come first.

The code:

function sumAll(arr) {
  var out = 0;
  for (var i = Math.min(...arr); i <= Math.max(...arr); i++) {
    out += i;
  }
  return out;
}

Test cases:

sumAll([1, 4])
sumAll([4, 1])
sumAll([5, 10])
sumAll([10, 5])

Expected output:

10
10
45
45

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
}

StackExchange Code Review Q#117166, answer score: 20