The MouseEvent.shiftKey read-only property is a boolean value that indicates whether the shift key was pressed or not when a given mouse event occurs.
A boolean value, where true indicates that the key is pressed, and false indicates that the key is not pressed.
This example logs the shiftKey property when you trigger a click event.
<p>Click anywhere to test the <code>shiftKey</code> property.</p>
<p id="log"></p>let log = document.querySelector("#log");
document.addEventListener("click", logKey);
function logKey(e) {
log.textContent = `The shift key is pressed: ${e.shiftKey}`;
}