static keyCodeNumber(event) {
        // `event.keyCode` and `event.which` are deprecated, `KeyboardEvent.key` (https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/key) must be used now
        // Also, do note that Firefox generate a 'keypress' event (e.keyCode === 0) for the keys that do not print a character (ie. 'Insert', 'Delete', 'Fn' keys, 'PageUp', 'PageDown' etc.). 'Shift' on the other hand does not generate a keypress event.
        return (typeof event.which === 'undefined')?event.keyCode:event.which;
    }
Return the code for the key used to generate the given event.

@param {Event} event
@returns {string|Number}