arraySum.js

function arraySum(i) {
  'use strict';
  function type(obj) {
    // Object.prototype.toString provides a way to reliably identify
    // builtin javascript types like Array, Number and String, but
    // will fail (in older browsers) if called on null or undefined
    //
    // The relevant specs:
    //
    //   http://es5.github.io/#x15.2.4.2
    //   http://ecma262-5.com/ELS5_Section_15.htm#Section_15.2.4.2
    //
    var _toString = Object.prototype.toString;
    return obj === null ? 'Null' 
      :    obj === undefined ? 'Undefined '
      :    _toString.call(obj).slice(8, -1);
  }

  function sum(obj) {
    switch (type(obj)) {
    case 'Number':
      // numbers sum to themselves
      return obj;

    case 'Array':
      // Arrays sum to the total of the sum of each item
      // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce
      return obj.reduce(function (total, obj) {
        // call sum on each element of obj
        return total + sum(obj);
      }, 0);

    default:
      // All other objects sum to 0
      return 0;
    }
  }

  return sum(i);
}