code: function findKey(object, callback, thisArg) {
        var result;
        callback = lodash.createCallback(callback, thisArg);
        forOwn(object, function(value, key, object) {
          if (callback(value, key, object)) {
            result = key;
            return false;
          }
        });
        return result;
      }
docs: This method is similar to `_.find`, except that it returns the key of the
  element that passes the callback check, instead of the element itself.
  
  @static
  @memberOf _
  @category Objects
  @param {Object} object The object to search.
  @param {Function|Object|String} [callback=identity] The function called per
  iteration. If a property name or object is passed, it will be used to create
  a "_.pluck" or "_.where" style callback, respectively.
  @param {Mixed} [thisArg] The `this` binding of `callback`.
  @returns {Mixed} Returns the key of the found element, else `undefined`.
  @example
  
  _.findKey({ 'a': 1, 'b': 2, 'c': 3, 'd': 4 }, function(num) {
  return num % 2 == 0;
  });
  // => 'b'