The charIndex read-only property of the SpeechSynthesisUtterance interface returns the index position of the character in SpeechSynthesisUtterance.text that was being spoken when the event was triggered.
A number.
utterThis.onpause = (event) => {
const char = event.utterance.text.charAt(event.charIndex);
console.log(
`Speech paused at character ${event.charIndex} of "${event.utterance.text}", which is "${char}".`,
);
};