code: function _isArrayIndex (target, key) {
      var n = +key;
  
      return Array.isArray(target) && n % 1 === 0 && !(n < 0 && _isEnumerable(target, key));
  }
docs: Accepts a target object and a key name and verifies that the target is an array and that
  the key is an existing index.
  @private
  @param {Object} target
  @param {String|Number} key
  @returns {Boolean}