深入理解JavaScript回调函数：原理、陷阱与最佳实践

//注意到click方法中是一个函数而不是一个变量
//它就是回调函数
$("#btn_1").click(function() {
    alert("Btn 1 Clicked");
});

var friends = ["Mike", "Stacy", "Andy", "Rick"];

friends.forEach(function (eachName, index){
    console.log(index + 1 + ". " + eachName); // 1. Mike, 2. Stacy, 3. Andy, 4. Rick
});

//匿名函数不会再参数中被执行
//这是一个回调函数
$("#btn_1").click(function(){
    alert("Btn 1 Clicked");
});

//全局变量
var allUserData = [];

//普通的logStuff函数，将内容打印到控制台
function logStuff (userData){
    if ( typeof userData === "string"){
        console.log(userData);
    } else if ( typeof userData === "object"){
        for(var item in userData){
            console.log(item + ": " + userData[item]);
        }
    }
}

//一个接收两个参数的函数，后面一个是回调函数
function getInput (options, callback){
    allUserData.push(options);
    callback(options);
}

//当我们调用getInput函数时，我们将logStuff作为一个参数传递给它
//因此logStuff将会在getInput函数内被回调（或者执行）
getInput({name:"Rich",speciality:"Javascript"}, logStuff);
//name:Rich
//speciality:Javascript

//全局变量
var generalLastName = "Cliton";

function getInput (options, callback){
    allUserData.push (options);
    //将全局变量generalLastName传递给回调函数
    callback(generalLastName,options);
}

function getInput(options, callback){
    allUserData.push(options);
    
    //确保callback是一个函数
    if(typeof callback === "function"){
        //调用它，既然我们已经确定了它是可调用的
        callback(options);
    }
}

//定义一个拥有一些属性和一个方法的对象 //我们接着将会把方法作为回调函数传递给另一个函数

var clientData = {
    id: 094545,
    fullName "Not Set",
    //setUsrName是一个在clientData对象中的方法
    setUserName: fucntion (firstName, lastName){
        //这指向了对象中的fullName属性
        this.fullName = firstName + " " + lastName;
    }
} 

function getUserInput(firstName, lastName, callback){
    //在这做些什么来确认firstName/lastName
    //现在存储names
    callback(firstName, lastName);
}

getUserInput("Barack","Obama",clientData.setUserName);
console.log(clientData,fullName);  //Not Set
//fullName属性将在window对象中被初始化     
console.log(window.fullName);  //Barack Obama

//注意到我们增加了新的参数作为回调对象，叫做“callbackObj”
function getUserInput(firstName, lastName, callback. callbackObj){
    //在这里做些什么来确认名字
    callback.apply(callbackObj, [firstName, lastName]);
}

//我们将clientData.setUserName方法和clientData对象作为参数，clientData对象会被Apply方法使用来设置this对象     
getUserName("Barack", "Obama", clientData.setUserName, clientData);

//clientData中的fullName属性被正确的设置
console.log(clientUser.fullName); //Barack Obama

function successCallback(){
    //在发送之前做点什么
}
  
function successCallback(){
//在信息被成功接收之后做点什么
}

function completeCallback(){
//在完成之后做点什么
}

function errorCallback(){
    //当错误发生时做点什么
}

$.ajax({
    url:"http://fiddle.jshell.net/favicon.png",
    success:successCallback,
    complete:completeCallback,
    error:errorCallback
});

var p_client = new Db('integration_tests_20', new Server("127.0.0.1", 27017, {}), 
                 {'pk':CustomPKFactory});
p_client.open(function(err, p_client) {
    p_client.dropDatabase(function(err, done) {
        p_client.createCollection('test_custom_key', function(err, collection) {
            collection.insert({'a':1}, function(err, docs) {
                collection.find({'_id':new ObjectID("aaaaaaaaaaaa")}, 
                function(err, cursor) {
                    cursor.toArray(function(err, items) {
                        test.assertEquals(1, items.length);
                        // Let's close the db
                        p_client.close();
                    });
                });
            });
        });
    });
});

//首先，创建通用诗的生成函数；它将作为下面的getUserInput函数的回调函数

function genericPoemMaker(name, gender) {
    console.log(name + " is finer than fine wine.");
    console.log("Altruistic and noble for the modern time.");
    console.log("Always admirably adorned with the latest style.");
    console.log("A " + gender + " of unfortunate tragedies who still manages a perpetual smile");
}

//callback，参数的最后一项，将会是我们在上面定义的genericPoemMaker函数
function getUserInput(firstName, lastName, gender, callback) {
    var fullName = firstName + " " + lastName;
    // Make sure the callback is a function
    if (typeof callback === "function") {
    // Execute the callback function and pass the parameters to it
    callback(fullName, gender);
    }
}

getUserInput("Michael", "Fassbender", "Man", genericPoemMaker);
// 输出
/* Michael Fassbender is finer than fine wine.
Altruistic and noble for the modern time.
Always admirably adorned with the latest style.
A Man of unfortunate tragedies who still manages a perpetual smile.
*/

unction greetUser(customerName, sex)  {
    var salutation  = sex && sex === "Man" ? "Mr." : "Ms.";
    console.log("Hello, " + salutation + " " + customerName);
}

// 将greetUser作为一个回调函数
getUserInput("Bill", "Gates", "Man", greetUser);

// 这里是输出
Hello, Mr. Bill Gates