window.onload = () => { play.onclick = (e) => { var u = new SpeechSynthesisUtterance(); u.text = 'Hello World. This is a large test.'; u.lang = 'en-US'; u.rate = 1.0; let boundaryIdx = 0; u.onboundary = function(event) { let word = u.text.substring(boundaryIdx, event.charIndex); boundaryIdx = event.charIndex; console.log('boundary', word, event); }; speechSynthesis.speak(u); }; };