var nums = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

  function sum_recurse(arr, i) {

   i = i || 0;

   if (i > arr.length)

     return 0;

   else

     return arr[i] + sum_recurse(arr, i + 1);

  }

​

 function sum_recurse_tail(arr, i, sum = 0) {

   i = i || 0;

   if (i > arr.length)

     return 0;

   else

     return sum_recurse(arr, i + 1, sum + arr[i]);

  }

  function sum_while(arr) {

   var total = 0,

       i = 0,

       len = arr.length;

   while (i < len) {

    total += arr[i++];

   }

   return total;

  }

  function sum_for(arr) {

   var total = 0,

       len = arr.length;

   for (var i = 0; i < len; i++) {

    total += arr[i];

   }

   return total;

  }