Prevent unexpected JS type coercion

We have the following JS:

// Allows ESC key to kill the throbber.
if (esc_key) {
  document.onkeydown = function(evt) {
    evt = evt || window.event;
    var isEscape = false;
    if ("key" in evt) {
      isEscape = evt.key == "Escape";
    } else {
      isEscape = evt.keyCode == 27;
    }
    if (isEscape) {
      $('.page-load-progress-lock-screen').remove();
    }
  };
}

Problem is evt.key and evt.keyCode can unexpectedly create JS type coercion due to the == comparison operator. Seems quite obvious for evt.keyCode to only require an integer but it's far less obvious exactly what to do for evt.key since you can have Escape, Esc or possibly other things. This will need some research, but the idea is to switch to using the === operator.