CaretPosition: offsetNode 属性

<input aria-label="text field" value="Click inside this input field" />

<pre id="log"></pre>

input {
  width: 100%;
  padding: 10px;
  font-size: 16px;
  box-sizing: border-box;
}

#log {
  height: 200px;
  overflow: scroll;
  padding: 0.5rem;
  border: 1px solid black;
}

document.querySelector("input").addEventListener("click", (event) => {
  const x = event.clientX;
  const y = event.clientY;

  const caret = document.caretPositionFromPoint?.(x, y);
  if (!caret) {
    log("Not supported");
    return;
  }

  const node = caret.offsetNode;
  const offset = caret.offset;

  log(`offsetNode: ${node}`);
  log(`offset: ${offset}`);
});

const logElement = document.querySelector("#log");
function log(text) {
  logElement.innerText = `${logElement.innerText}${text}\n`;
  logElement.scrollTop = logElement.scrollHeight;
}