function array_reduce (a_input, callback) {
  // http://kevin.vanzonneveld.net
  // +   original by: Alfonso Jimenez (http://www.alfonsojimenez.com)
  // %        note 1: Takes a function as an argument, not a function's name
  // *     example 1: array_reduce([1, 2, 3, 4, 5], function (v, w){v += w;return v;});
  // *     returns 1: 15
  var lon = a_input.length;
  var res = 0,
    i = 0;
  var tmp = [];


  for (i = 0; i < lon; i += 2) {
    tmp[0] = a_input[i];
    if (a_input[(i + 1)]) {
      tmp[1] = a_input[(i + 1)];
    } else {
      tmp[1] = 0;
    }
    res += callback.apply(null, tmp);
    tmp = [];
  }

  return res;
}

array_reduce([1, 2, 3, 4, 5], function (v, w){v += w;return v;});

15