Table of contents of this article:
In JavaScript, function is a built-in class object, which means it is a type of object that can be used for the management of built-in objects like other String, Array, Number, and Object class objects. Because a function is actually an object, it can be "stored in a variable, passed to (another) function through parameters, created within a function, and returned as a result value from the function."
Because function is a built-in object, we can pass it as a parameter to another function, delay execution in the function, or even return it after execution. This is the essence of using callback functions in JavaScript. The remainder of this article will be a comprehensive study of JavaScript callback functions. Callback functions may be the most widely used functional programming technology in JavaScript. Perhaps just a small piece of JavaScript or jQuery code will leave developers with a sense of mystery. After reading this article, it may help you eliminate this sense of mystery. .
The callback function comes from a well-known programming paradigm - functional programming . At a basic level, functional programming specifies the parameters of the function. Although functional programming is now used in a smaller scope, it has always been regarded as a difficult technology by "professional smart" programmers. This has been the case in the past and will be so in the future.
Fortunately, functional programming has been explained in a way that ordinary people like you and me can understand and use it. One of the most important techniques of functional programming is the callback function. As you will soon read, implementing the callback function is as simple as passing ordinary parameter variables. This technique is so simple that I wonder why it's often included in advanced JavaScript topics.
1. What is a callback or advanced function?
The callback function is considered a high-level function, a high-level function that is passed as a parameter to another function (called "otherFunction" here). The callback function will be called (or executed) within otherFunction. The essence of the callback function is a pattern (a pattern for solving common problems), so the callback function is also called the callback pattern.
Think about the following callback functions commonly used in jQuery:
//Note that the item in the click method's parameter is a function, not a variable.
//The item is a callback function
$("#btn_1").click(function() {
alert("Btn 1 Clicked");
});
As seen in the previous example, we passed a function to the formal parameter of the click method, and the click method will call (or execute) the callback function we passed to it. This example gives a typical way of using callback functions in JavaScript, and is widely used in jQuery.
Let’s take a closer look at another typical example of basic JavaScript:
var friends = ["Mike", "Stacy", "Andy", "Rick"];
friends.forEach(function (eachName, index){
console.log(index + 1 + ". " + eachName); // 1. Mike, 2. Stacy, 3. Andy, 4. Rick
});
We once again pass an anonymous function (a function without a function name) to the forEach method in the same way as the parameter of forEach.
So far, we have passed an anonymous function as a parameter to another function or method. Before looking at other more complex callback functions, let's understand how callbacks work and implement one of our own.
2. How is the callback function implemented?
We can use a function like a variable, as a parameter of another function, as a return result in another function, and call it in another function. When we pass a callback function as a parameter to another function, we only pass the definition of the function and do not execute it in the parameters.
When the containing (calling) function has a callback function defined in the parameter, it can call (that is, callback) it at any time.
This shows that the callback function is not executed immediately, but "calls back" it at a specified position in the function body containing the function (as the name suggests). So, even the first jQuery example looks like this:
//The anonymous function is not being executed there in the parameter.
//The item is a callback function
$("#btn_1").click(function() {
alert("Btn 1 Clicked");
});
匿名函数将延迟在click函数的函数体内被调用，即使没有名称，也可以被包含函数通过 arguments对象访问。
回调函数是闭包的
当作为参数传递一个回调函数给另一个函数时，回调函数将在包含函数函数体内的某个位置被执行，就像回调函数在包含函数的函数体内定义一样。这意味着回调函数是闭包的，想更多地了解闭包，请参考作者另一个贴子Understand JavaScript Closures With Ease。从所周知，闭包函数可以访问包含函数的作用域，所以，回调函数可以访问包含函数的变量，甚至是全局变量。
三、实现回调函数的基本原则
简单地说，自己实现回调函数的时候需要遵循几条原则。
1、使用命名函数或匿名函数作为回调
在前面的jQuery和forEach的例子中，我们在包含函数的参数中定义匿名函数，这是使用回调函数的通用形式之一，另一个经常被使用的形式是定义一个带名称的函数，并将函数名作为参数传递给另一个函数，例如：

// global variable
var allUserData = [];
// generic logStuff function that prints to console
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]);
}
}
}
// A function that takes two parameters, the last one a callback function
function getInput (options, callback) {
allUserData.push (options);
callback (options);
}
// When we call the getInput function, we pass logStuff as a parameter.
// So logStuff will be the function that will called back (or executed) inside the getInput function
getInput ({name:"Rich", speciality:"JavaScript"}, logStuff);
// name: Rich
// speciality: JavaScript
2、传递参数给回调函数
因为回调函数在执行的时候就和一般函数一样，我们可以传递参数给它。可以将包含函数的任何属性(或全局的属性)作为参数传递回调函数。在上一个例子中，我们将包含函数的options作为参数传递给回调函数。下面的例子让我们传递一个全局变量或本地变量给回调函数：
//Global variable
var generalLastName = "Clinton";
function getInput (options, callback) {
allUserData.push (options);
// Pass the global variable generalLastName to the callback function
callback (generalLastName, options);
}
3、在执行之前确保回调是一个函数
在调用之前，确保通过参数传递进来的回调是一个需要的函数通常是明智的。此外，让回调函数是可选的也是一个好的实践。
让我们重构一下上面例子中的getInput函数，确保回调函数做了适当的检查。
function getInput(options, callback) {
allUserData.push(options);
// Make sure the callback is a function
if (typeof callback === "function") {
// Call it, since we have confirmed it is callable
callback(options);
}
}
如果getInput函数没有做适当的检查(检查callback是否是函数，或是否通过参数传递进来了),我们的代码将会导致运行时错误。
4、使用含有this对象的回调函数的问题
当回调函数是一个含有this对象的方法时，我们必须修改执行回调函数的方法以保护this对象的内容。否则this对象将会指向全局的window对象(如果回调函数传递给了全局函数)，或指向包含函数。让我们看看下面的代码：
// Define an object with some properties and a method
// We will later pass the method as a callback function to another function
var clientData = {
id: 094545,
fullName: "Not Set",
// setUserName is a method on the clientData object
setUserName: function (firstName, lastName) {
// this refers to the fullName property in this object
this.fullName = firstName + " " + lastName;
}
}
function getUserInput(firstName, lastName, callback) {
// Do other stuff to validate firstName/lastName here
// Now save the names
callback (firstName, lastName);
}
在下面的示例代码中，当clientData.setUserName被执行时，this.fullName不会设置clientData 对象中的属性fullName，而是设置window 对象中的fullName，因为getUserInput是一个全局函数。出现这种现象是因为在全局函数中this对象指向了window对象。
getUserInput ("Barack", "Obama", clientData.setUserName);
console.log (clientData.fullName);// Not Set
// The fullName property was initialized on the window object
console.log (window.fullName); // Barack Obama
5、使用Call或Apply函数保护this对象
我们可以通过使用 Call 或 Apply函数来解决前面示例中的问题。到目前为止，我们知道JavaScript中的每一个函数都有两个方法:Call和Apply。这些方法可以被用来在函数内部设置this对象的内容，并内容传递给函数参数指向的对象。
Call takes the value to be used as the this object inside the function as the first parameter, and the remaining arguments to be passed to the function are passed individually (separated by commas of course). The Apply function's first parameter is also the value to be used as the thisobject inside the function, while the last parameter is an array of values (or the arguments object) to pass to the function. (该段翻译起来太拗口了，放原文自己体会)
这听起来很复杂，但让我们看看Apply和Call的使用是多么容易。为解决前面例子中出现的问题，我们使用Apply函数如下：
//Note that we have added an extra parameter for the callback object, called "callbackObj"
function getUserInput(firstName, lastName, callback, callbackObj) {
// Do other stuff to validate name here
// The use of the Apply function below will set the this object to be callbackObj
callback.apply (callbackObj, [firstName, lastName]);
}
通过Apply函数正确地设置this对象，现在我们可以正确地执行回调函数并它正确地设置clientData对象中的fullName属性。
// We pass the clientData.setUserName method and the clientData object as parameters. The clientData object will be used by the Apply function to set the this object

getUserInput ("Barack", "Obama", clientData.setUserName, clientData);
// the fullName property on the clientData was correctly set
console.log (clientData.fullName); // Barack Obama
我们也可以使用Call 函数，但在本例中我们使用的Apply 函数。
6、多重回调函数也是允许的
我们可以传递多个回调函数给另一个函数，就像传递多个变量一样。这是使用jQuery的AJAX函数的典型例子：
function successCallback() {
// Do stuff before send
}
function successCallback() {
// Do stuff if success message received
}
function completeCallback() {
// Do stuff upon completion
}
function errorCallback() {
// Do stuff if error received
}
$.ajax({
url:"http://fiddle.jshell.net/favicon.png",
success:successCallback,
complete:completeCallback,
error:errorCallback
});
四、“回调地狱”的问题和解决方案
异步代码执行是一种简单的以任意顺序执行的方式，有时是很常见的有很多层级的回调函数，你看起来像下面这样的代码。下面这种凌乱的代码称作“回调地狱”，因为它是一种包含非常多的回调的麻烦的代码。我是在node-mongodb-native里看到这个例子的，MongoDB驱动Node.js.示例代码就像这样：
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();
});
});
});
});
});
});
你不太可能在自己的代码里碰到这个的问题，但如果你碰到了(或以后偶然碰到了)，那么有以下两种方式解决这个问题。
命名并定义你的函数，然后传递函数名作为回调，而不是在主函数的参数列表里定义一个匿名函数。
模块化：把你的代码划分成一个个模块，这样你可以空出一部分代码块做特殊的工作。然后你可以将这个模型引入到你的大型应用程序中。
五、实现自己的回调函数
现在你已经完全理解(我相信你已经理解了，如果没有请快速重新阅读一遍)了JavaScript关于回调的所用特性并且看到回调的使用是如此简单但功能却很强大。你应该看看自己的代码是否有机会使用回调函数，有以下需求时你可以考虑使用回调：
实现自己的回调函数很简单，在下面的例子中，我可以创建一个函数完成所用的工作：获取用户数据，使用用户数据生成一首通用的诗，使用用户数据来欢迎用户，但这个函数将会是一个凌乱的函数，到处是if/else的判断，甚至会有很多的限制并无法执行应用程序可能需要的处理用户数据的其它函数。
替而代之的是我让实现增加了回调函数，这样主函数获取用户数据后可以传递用户全名和性别给回调函数的参数并执行回调函数以完成任何任务。
简而言之，getUserInput函数是通用的，它可以执行多个拥有各种功能的回调函数。
// First, setup the generic poem creator function; it will be the callback function in the getUserInput function below.
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");
}
//The callback, which is the last item in the parameter, will be our genericPoemMaker function we defined above.
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);
}
}
调用getUserInput函数并传递genericPoemMaker函数作为回调：
getUserInput("Michael", "Fassbender", "Man", genericPoemMaker);
// Output
/* 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.
*/因为getUserInput 函数只处理用户数据的输入，我们可以传递任何回调函数给它。例如我们可以像这样传递一个greetUser函数。
function greetUser(customerName, sex) {
var salutation = sex && sex === "Man" ? "Mr." : "Ms.";
console.log("Hello, " + salutation + " " + customerName);
}
// Pass the greetUser function as a callback to getUserInput
getUserInput("Bill", "Gates", "Man", greetUser);
// And this is the output
Hello, Mr. Bill Gates
和上一个例子一样，我们调用了同一个getUserInput 函数，但这次却执行了完全不同的任务。
如你所见，回调函数提供了广泛的功能。尽管前面提到的例子非常简单，在你开始使用回调函数的时候思考一下你可以节省多少工作，如何更好地抽象你的代码。加油吧！在早上起来时想一想，在晚上睡觉前想一想，在你休息时想一想……
我们在JavaScript中经常使用回调函数时注意以下几点，尤其是现在的web应用开发，在第三方库和框架中
以上就是更加深入的学习了JavaScript的回调函数，希望对大家的学习有所帮助。
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