keyboardDown = function (key, callback) {
        var key_pressed;

        if (key.keyCode in keysDown) {
           return;
        }

       keysDown[key.keyCode] = true;

       if (typeof key_map[key.keyCode] !== 'undefined') {
            key_pressed = getKeyPressed(key.keyCode);

            // Call user's noteDown function.
            callback(key_pressed, getFrequencyOfNote(key_pressed));
            lightenUp(document.getElementById(key_pressed));
       }
    }
Handle a keyboard key being pressed.
@param {object} key The keyboard event of the currently pressed key.
@param {callback} callback The user's noteDown function.