Ever since serializer was deprecated in favor of toJSON [1], the Performance and PerformanceEntry specs added a mandatory toJSON method. This method should just call the default toJSON operation [2]. [1] https://github.com/w3c/resource-timing/issues/112 [2] https://heycam.github.io/webidl/#default-tojson-operation The following interfaces will have toJSON added: PerformanceResourceTiming PerformanceLongTaskTiming TaskAttributionTiming