Basic implementation of Array.prototype.inject() in Ruby-like method in Javascript

injectable.spec.js

Array.prototype.myinject = function () {
  var obj = Object(this);
  var base;
  var callback;

  if (arguments.length === 2) {
    base = arguments[0];
    callback = arguments[1];
  } else if (arguments.length === 1) {
    callback = arguments[0];
  } else {
    return null;
  }

  if (typeof(callback) === 'undefined') return null;
  if (obj.length === 0) return null;

  switch (typeof(obj[0])) {
  case 'number':
    base = 0;
    break;
  case 'string':
    base = '';
    break;
  default:
    return null;
  }

  for (var i = 0, o; o = obj[i]; i++) {
    base = callback(base, o);
  }

  return base;
};

describe('an Array', function () {
  describe('#myinject', function () {
    var arr;
    var arr2;

    beforeEach(function () {
      arr = [1, 2, 3, 4, 5];
      arr2 = ['a', 'b', 'c'];
    });

    it('should return nil if no block exists', function () {
      expect(arr.myinject()).toBe(null);
    });

    it('should return return 15 for accumulator + element', function () {
      expect(arr.myinject(0, function (acc, e) { return acc + e })).toBe(15);
      expect(arr.myinject(function (acc, e) { return acc + e })).toBe(15);
    });

    it('should return aabaca for accumulator + element + a', function () {
      expect(arr2.myinject('', function (acc, e) { return acc + e + 'a' })).toBe('aabaca');
      expect(arr2.myinject(function (acc, e) { return acc + e + 'a' })).toBe('aabaca');
    });
  });
});