_.assignInWith(object, sources, [customizer])
functioncustomizer(objValue, srcValue) { return_.isUndefined(objValue) ?srcValue : objValue; } vardefaults=_.partialRight(_.assignInWith, customizer); defaults({ 'a': 1 }, { 'b': 2 }, { 'a': 3 }); // => { 'a': 1, 'b': 2 }