lassList.add('pressed');
              playFrequency(frequency);
            };
            const keyReleased = (e) => {
              domElement.classList.remove('pressed');
              muteFrequency(frequency);
            };
            ['mousedown', 'mouseover', 'touchstart', 'touchenter']
              .forEach(event => domElement.addEventListener(event, keyPressed, false));
            ['mouseup', 'mouseleave', 'touchend', 'touchleave']
              .forEach(event => domElement.addEventListener(eve