SpeechSynthesis.cancel()

cancel()

var synth = window.speechSynthesis;

var utterance1 = new SpeechSynthesisUtterance('How about we say this now? This is quite a long sentence to say.');
var utterance2 = new SpeechSynthesisUtterance('We should say another sentence too, just to be on the safe side.');

synth.speak(utterance1);
synth.speak(utterance2);

synth.cancel(); // utterance1 stops being spoken immediately, and both are removed from the queue