InputEvent: getTargetRanges() メソッド

getTargetRanges()

function isBeforeInputEventAvailable() {
  return (
    window.InputEvent &&
    typeof InputEvent.prototype.getTargetRanges === "function"
  );
}

const editableElem = document.querySelector('[contenteditable="true"]');
editableElem.addEventListener("beforeinput", (e) => {
  const targetRanges = e.getTargetRanges();
  console.log(targetRanges);
});