問(wèn)題描述

再看JavaScript異步編程這本書(shū)，然后看到了一段代碼

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(shū)中說(shuō)

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

這樣會(huì)使代碼崩潰，不解。。在返回值之前調(diào)用回調(diào)函數(shù)為什么會(huì)使代碼崩潰。希望大大們能幫我解釋解釋

問(wèn)題解答

在返回之前回調(diào)函數(shù)就有可能被執(zhí)行了，這個(gè)時(shí)候的socket還沒(méi)有賦值

可以給callback傳一個(gè)參數(shù)來(lái)避免這種情況

你url 定義了么- -

const func = function (callback) { callback(); return 100;};const x = func(() => { console.log(x); //此處將打印 undefined;});console.log(x); //此處打印 100

這樣解釋不知道你能否明白？