再看JavaScript非同步程式設計這本書，然後看到了一段程式碼

var webSocketCache = {};
function openWebSocket(serverAddress, callback) {
    var socket;
    if (serverAddress in webSocketCache) {
        socket = webSocketCache[serverAddress];
        if (socket.readyState === WebSocket.OPEN) {
            callback();
        } else {
            socket.onopen = _.compose(callback, socket.onopen);
        };
    } else {
        socket = new WebSocket(serverAddress);
        webSocketCache[serverAddress] = socket;
        socket.onopen = callback;
    };
    return socket;
};

書中說

        var socket=openWebSocket(url,function(){
          socket.send('Hello,server!');      
        });

這樣會使程式碼崩潰，不解。 。在返回值之前調用回調函數為什麼會使程式碼崩潰。希望大佬們能幫我解釋解釋