The KeyboardEvent.altKey read-only property is a boolean value that indicates if the alt key (Option or ⌥ on macOS) was pressed (true) or not (false) when the event occurred.
A boolean value.
<p>
Press any character key, with or without holding down the ALT key.<br />
You can also use the SHIFT key together with the ALT key.
</p>
<pre id="output"></pre>const output = document.getElementById("output");
function showChar(e) {
output.textContent = `Key KeyDown: "${e.key}"
ALT key KeyDown: ${e.altKey}
`;
}
document.addEventListener("keydown", showChar);