问jQuery未按预期执行函数

EN

$(document).ready(function() {
    $('#show-video').click(function(event) {
        jQuery('.player-hold').slideDown("slow");
        $('#book').slideDown('slow', function() {
            playVideo();
        });
        jQuery('.features').slideUp("slow");
    }); 
});

function onPlayerStateChange(newState) {
    if(newState == 0) {
        jQuery('.player-hold').slideUp("slow");
        jQuery('.features').slideDown("slow");
    }
}

function playVideo() {
    if (ytplayer) {
        ytplayer.playVideo();
    }
}

function onYouTubePlayerReady(playerId) {
    ytplayer = document.getElementById("ytPlayer");
    ytplayer.addEventListener("onStateChange", "onPlayerStateChange");
    ytplayer.setPlaybackQuality("highres");
    ytplayer.cueVideoById("PoTa2FTmDWk");
}