Read the book JavaScript Asynchronous Programming again, and then I saw a piece of code

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;
};

The book says

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

This will cause the code to crash, which is confusing. . Why does calling a callback function before returning a value crash the code. I hope you guys can help me explain