Definition
Web Speech API defines SpeechSynthesisEvent
[Exposed=Window]
interface SpeechSynthesisEvent : Event {
constructor(DOMString type, SpeechSynthesisEventInit eventInitDict);
readonly attribute SpeechSynthesisUtterance utterance;
readonly attribute unsigned long charIndex;
readonly attribute unsigned long charLength;
readonly attribute float elapsedTime;
readonly attribute DOMString name;
};