Selection: direction 属性

<p id="text-box">
  Select text in this paragraph to see the selection direction.
</p>

#log {
  height: 50px;
  padding: 0.5rem;
  margin: 5px;
  border: 1px solid black;
}

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

const logElement = document.querySelector("#log");

function log(text) {
  logElement.innerText = text;
}

const input = document.getElementById("text-box");

if ("direction" in Selection.prototype) {
  document.addEventListener("selectionchange", () => {
    const selection = window.getSelection();
    log(`Selection direction: ${selection.direction}`);
  });
} else {
  log("direction property not defined");
  input.hidden = true;
}