Sum all numbers between two integers

Question

OBJECTIVE

Given two numbers in an array, sum all the numbers including (and between) both integers (e.g [4,2] -> 2 + 3 + 4 = 9).

I've managed to solve the question but was wondering if there is a more elegant solution (especially using Math.max and Math.min) - see below for more questions...

MY SOLUTION

//arrange array for lowest to highest number
function order(min,max) {
  return min - max;
}


function sumAll(arr) {
  var list = arr.sort(order);
  var a = list[0]; //smallest number
  var b = list[1]; //largest number
  var c = 0;

  while (a <= b) {
    c = c + a; //add c to itself
    a += 1; // increment a by one each time
  }

  return c;
}

sumAll([10, 5]);

MY QUESTION(S)

1. Is there a more efficient way to do this?
2. How would I use Math.max() and Math.min() for an array?

Answer 1

Optimum algorithm

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