Document: queryCommandState() メソッド

queryCommandState(command)

<div contenteditable="true">Select a part of this text!</div>
<button onclick="makeBold();">Test the state of the 'bold' command</button>
<hr />
<div id="output"></div>

hr,
button {
  margin: 1rem 0;
}

function makeBold() {
  const state = document.queryCommandState("bold");
  let message;
  switch (state) {
    case true:
      message = "The bold formatting will be removed from the selected text.";
      break;
    case false:
      message = "The selected text will be displayed in bold.";
      break;
    default:
      message = "The state of the 'bold' command is indeterminable.";
      break;
  }
  document.querySelector("#output").textContent = `Output: ${message}`;
  document.execCommand("bold");
}