Sum All Numbers in a Range, Freecodecamp challenge, URL: https://www.freecodecamp.com/challenges/sum-all-numbers-in-a-range

sum_of_all_numbers_in_a_range.js

function sumAll(arr) { 
  var min = Math.min(arr[0] , arr[1]);
  var max = Math.max(arr[0] , arr[1]);
  var sum = 0;
  for(var i=min ; i<=max ; i++) {
    sum+=i; 
  }
  return sum;
}

sum_of_all_numbers_in_a_range_math_solution.js

function sumAll(arr) { 
  var min = Math.min( arr[0] , arr[1]);
  var max = Math.max( arr[0] , arr[1]);
  return ((max*(max+1))/2-(min*(min-1))/2);
}
sumAll([1, 4]);