本文翻译自：How to access the correct `this` inside a callback?
I have a constructor function which registers an event handler: 我有一个构造函数注册一个事件处理程序：
function MyConstructor(data, transport) { this.data = data; transport.on('data', function () { alert(this.data); }); } // Mock transport object var transport = { on: function(event, callback) { setTimeout(callback, 1000); } }; // called as var obj = new MyConstructor('foo', transport);
However, I'm not able to access the data property of the created object inside the callback. 但是，我无法在回调内部访问已创建对象的data属性。 It looks like this does not refer to the object that was created but to an other one. 看起来this并不引用创建的对象，而是引用另一个对象。
I also tried to use an object method instead of an anonymous function: 我还尝试使用对象方法而不是匿名函数：
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', this.alert);
}
MyConstructor.prototype.alert = function() {
alert(this.name);
};
but it exhibits the same problems. 但是也有同样的问题
How can I access the correct object? 如何访问正确的对象？
#1楼
参考：https://stackoom.com/question/1N5cS/如何在回调中访问正确的-this
#2楼
What you should know about this 你应该知道什么this
this (aka "the context") is a special keyword inside each function and its value only depends on how the function was called, not how/when/where it was defined. this （又名“上下文”）是每个函数里面一个特殊的关键字，它的值只取决于函数是怎么被调用，而不是如何/何时/何地它被定义。 It is not affected by lexical scopes like other variables (except for arrow functions, see below). 它不受其他变量之类的词法作用域的影响（箭头函数除外，请参见下文）。 Here are some examples: 这里有些例子：
function foo() {
console.log(this);
}
// normal function call
foo(); // `this` will refer to `window`
// as object method
var obj = {bar: foo};
obj.bar(); // `this` will refer to `obj`
// as constructor function
new foo(); // `this` will refer to an object that inherits from `foo.prototype`
To learn more about this , have a look at the MDN documentation . 要了解更多关于this ，看看在MDN文档 。
How to refer to the correct this 如何引用正确的this
Don't use this 不要用this
You actually don't want to access this in particular, but the object it refers to . 实际上，您实际上不想访问this 对象 ，但是要访问它所指向的对象 。 That's why an easy solution is to simply create a new variable that also refers to that object. 这就是为什么一个简单的解决方案是简单地创建一个也引用该对象的新变量。 The variable can have any name, but common ones are self and that . 该变量可以具有任何名称，但是常见的名称是self和that 。
function MyConstructor(data, transport) {
this.data = data;
var self = this;
transport.on('data', function() {
alert(self.data);
});
}
Since self is a normal variable, it obeys lexical scope rules and is accessible inside the callback. 由于self是一个普通变量，因此它遵循词汇范围规则，并且可以在回调内部进行访问。 This also has the advantage that you can access the this value of the callback itself. 这还有一个优点，就是您可以访问回调本身的this值。
Explicitly set this of the callback - part 1 明确设置this回调-第1部分
It might look like you have no control over the value of this because its value is set automatically, but that is actually not the case. 它看起来像你有过的价值无法控制this ，因为它的值是自动设置的，但实际上并非如此。
Every function has the method .bind [docs] , which returns a new function with this bound to a value. 每个函数都有所述方法.bind [文档] ，它返回一个新的功能this绑定到一个值。 The function has exactly the same behaviour as the one you called .bind on, only that this was set by you. 该功能具有完全相同的行为，你那叫一个.bind上，只有this被你设置。 No matter how or when that function is called, this will always refer to the passed value. 无论如何或何时调用该函数， this始终引用传递的值。
function MyConstructor(data, transport) {
this.data = data;
var boundFunction = (function() { // parenthesis are not necessary
alert(this.data); // but might improve readability
}).bind(this); // <- here we are calling `.bind()`
transport.on('data', boundFunction);
}
In this case, we are binding the callback's this to the value of MyConstructor 's this . 在这种情况下，我们绑定回调就是this以价值MyConstructor的this 。
Note: When binding context for jQuery, use jQuery.proxy [docs] instead. 注意：当为jQuery绑定上下文时，请改用jQuery.proxy [docs] 。 The reason to do this is so that you don't need to store the reference to the function when unbinding an event callback. 这样做的原因是，使您在取消绑定事件回调时不需要存储对该函数的引用。 jQuery handles that internally. jQuery在内部进行处理。
ECMAScript 6: Use arrow functions ECMAScript 6：使用箭头功能
ECMAScript 6 introduces arrow functions , which can be thought of as lambda functions. ECMAScript 6引入了箭头功能 ，可以将其视为lambda函数。 They don't have their own this binding. 自己他们没有this约束力。 Instead, this is looked up in scope just like a normal variable. 相反， this像普通变量一样在作用域中查找。 That means you don't have to call .bind . 这意味着您不必调用.bind 。 That's not the only special behaviour they have, please refer to the MDN documentation for more information. 这不是它们唯一的特殊行为，请参考MDN文档以获取更多信息。
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => alert(this.data));
}
Set this of the callback - part 2 设置this回调-第2部分
Some functions/methods which accept callbacks also accept a value to which the callback's this should refer to. 其中接受回调的某些功能/方法也接受一个值，其回调是this应该是指。 This is basically the same as binding it yourself, but the function/method does it for you. 这基本上与您自己绑定它相同，但是函数/方法可以为您完成它。 Array#map [docs] is such a method. Array#map [docs]是这种方法。 Its signature is: 它的签名是：
array.map(callback[, thisArg])
The first argument is the callback and the second argument is the value this should refer to. 第一个参数是回调，第二个参数是值this应该参考。 Here is a contrived example: 这是一个人为的示例：
var arr = [1, 2, 3];
var obj = {multiplier: 42};
var new_arr = arr.map(function(v) {
return v * this.multiplier;
}, obj); // <- here we are passing `obj` as second argument
Note: Whether or not you can pass a value for this is usually mentioned in the documentation of that function/method. 注意：该函数/方法的文档中通常会提到是否可以this传递值。 For example, jQuery's $.ajax method [docs] describes an option called context : 例如， jQuery的$.ajax方法[docs]描述了一个称为context的选项：
This object will be made the context of all Ajax-related callbacks. 该对象将成为所有与Ajax相关的回调的上下文。
Common problem: Using object methods as callbacks/event handlers 常见问题：使用对象方法作为回调/事件处理程序
Another common manifestation of this problem is when an object method is used as callback/event handler. 此问题的另一个常见表现是将对象方法用作回调/事件处理程序。 Functions are first-class citizens in JavaScript and the term "method" is just a colloquial term for a function that is a value of an object property. 函数是JavaScript中的一等公民，术语“方法”仅是一个俗称的函数，即对象属性的值。 But that function doesn't have a specific link to its "containing" object. 但是该函数没有指向其“包含”对象的特定链接。
Consider the following example: 考虑以下示例：
function Foo() {
this.data = 42,
document.body.onclick = this.method;
}
Foo.prototype.method = function() {
console.log(this.data);
};
The function this.method is assigned as click event handler, but if the document.body is clicked, the value logged will be undefined , because inside the event handler, this refers to the document.body , not the instance of Foo . 函数this.method被分配为click事件处理程序，但是如果单击document.body ，则记录的值将是undefined ，因为在事件处理程序内部， this引用document.body ，而不是Foo的实例。
As already mentioned at the beginning, what this refers to depends on how the function is called , not how it is defined . 正如开头已经提到的，什么this是指依赖于该函数的调用 ，它不是如何定义的 。
If the code was like the following, it might be more obvious that the function doesn't have an implicit reference to the object: 如果代码如下所示，则该函数没有对该对象的隐式引用可能会更加明显：
function method() {
console.log(this.data);
}
function Foo() {
this.data = 42,
document.body.onclick = this.method;
}
Foo.prototype.method = method;
The solution is the same as mentioned above: If available, use .bind to explicitly bind this to a specific value 如果可能，使用：如上面提到的解决方案是相同.bind将其明确绑定this在某一特定值
document.body.onclick = this.method.bind(this);
or explicitly call the function as a "method" of the object, by using an anonymous function as callback / event handler and assign the object ( this ) to another variable: 或通过将匿名函数用作回调/事件处理程序并将对象（ this ）分配给另一个变量，来显式调用该函数作为对象的“方法”：
var self = this;
document.body.onclick = function() {
self.method();
};
or use an arrow function: 或使用箭头功能：
document.body.onclick = () => this.method();
#3楼
It's all in the "magic" syntax of calling a method: 所有这些都是调用方法的“魔术”语法：
object.property();
When you get the property from the object and call it in one go, the object will be the context for the method. 当您从对象获得属性并一次性调用它时，该对象将成为方法的上下文。 If you call the same method, but in separate steps, the context is the global scope (window) instead: 如果您调用相同的方法，但在单独的步骤中，则上下文将改为全局作用域（窗口）：
var f = object.property;
f();
When you get the reference of a method, it's no longer attached to the object, it's just a reference to a plain function. 当您获得方法的引用时，它不再附加到对象上，而只是对普通函数的引用。 The same happens when you get the reference to use as a callback: 当您将引用用作回调时，也会发生相同的情况：
this.saveNextLevelData(this.setAll);
That's where you would bind the context to the function: 那是将上下文绑定到函数的地方：
this.saveNextLevelData(this.setAll.bind(this));
If you are using jQuery you should use the $.proxy method instead, as bind is not supported in all browsers: 如果您使用的是jQuery，则应改用$.proxy方法，因为并非所有浏览器都支持bind ：
this.saveNextLevelData($.proxy(this.setAll, this));
#4楼
The trouble with "context" 麻烦与“上下文”
The term "context" is sometimes used to refer to the object referenced by this . 术语“上下文”有时用于表示this引用的对象。 Its use is inappropriate because it doesn't fit either semantically or technically with ECMAScript's this . 它的使用是不合适的，因为它在语义上或技术上都不适合ECMAScript的this 。
"Context" means the circumstances surrounding something that adds meaning, or some preceding and following information that gives extra meaning. “上下文”是指围绕某些事物增加含义的环境，或一些提供额外含义的前后信息。 The term "context" is used in ECMAScript to refer to execution context , which is all the parameters, scope and this within the scope of some executing code. 术语“上下文” ECMAScript中用于指代执行上下文 ，这是所有的参数，范围和这个的一些执行的代码的范围内。
This is shown in ECMA-262 section 10.4.2 : 这在ECMA-262第10.4.2节中显示 ：
Set the ThisBinding to the same value as the ThisBinding of the calling execution context 将ThisBinding设置为与调用执行上下文的ThisBinding相同的值
which clearly indicates that this is part of an execution context. 这清楚地表明这是执行上下文的一部分。
An execution context provides the surrounding information that adds meaning to code that is being executed. 执行上下文提供了周围的信息，这些信息为正在执行的代码增加了含义。 It includes much more information than just the thisBinding . 它不仅包含thisBinding ，还包含更多信息。
So the value of this isn't "context", it's just one part of an execution context. 因此， 此值不是“上下文”，而只是执行上下文的一部分。 It's essentially a local variable that can be set by the call to any object and in strict mode, to any value at all. 它本质上是一个局部变量，可以通过对任何对象的调用以及在严格模式下将其设置为所有值。
#5楼
Here are several ways to access parent context inside child context - 以下是在子上下文中访问父上下文的几种方法-
- You can use
bind ()function. 您可以使用bind ()函数。 - Store reference to context/this inside another variable(see below example). 将对上下文/ this的引用存储在另一个变量中（请参见下面的示例）。
- Use ES6 Arrow functions. 使用ES6 箭头功能。
- Alter code/function design/architecture - for this you should have command over design patterns in javascript. 更改代码/功能设计/架构-为此，您应该对javascript中的设计模式有命令。
1. Use bind() function 1.使用bind()函数
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', ( function () {
alert(this.data);
}).bind(this) );
}
// Mock transport object
var transport = {
on: function(event, callback) {
setTimeout(callback, 1000);
}
};
// called as
var obj = new MyConstructor('foo', transport);
If you are using underscore.js - http://underscorejs.org/#bind 如果您使用的是underscore.js - http://underscorejs.org/#bind
transport.on('data', _.bind(function () {
alert(this.data);
}, this));
2 Store reference to context/this inside another variable 2将对上下文/ this的引用存储在另一个变量中
function MyConstructor(data, transport) {
var self = this;
this.data = data;
transport.on('data', function() {
alert(self.data);
});
}
3 Arrow function 3箭头功能
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => {
alert(this.data);
});
}
#6楼
First, you need to have a clear understanding of scope and behaviour of this keyword in the context of scope . 首先，您需要对scope和this关键字在scope上下文中的行为有清楚的了解。
this & scope : this和scope ：
there are two types of scope in javascript. They are :
1) Global Scope
2) Function Scope
in short, global scope refers to the window object.Variables declared in a global scope are accessible from anywhere.On the other hand function scope resides inside of a function.variable declared inside a function cannot be accessed from outside world normally. 简而言之，全局作用域是指window对象。在全局作用域中声明的变量可以从任何地方访问;另一方面，函数作用域位于函数内部。在函数内部声明的变量通常不能从外部访问。 this keyword in global scope refers to the window object. 全局范围内的this关键字引用窗口对象。 this inside function also refers to the window object.So this will always refer to the window until we find a way to manipulate this to indicate a context of our own choosing. this内部函数也指的是窗口对象， this它将一直指代窗口，直到我们找到一种方法来操纵this以表明我们自己选择的上下文为止。
--------------------------------------------------------------------------------
- -
- Global Scope -
- ( globally "this" refers to window object) -
- -
- function outer_function(callback){ -
- -
- // outer function scope -
- // inside outer function"this" keyword refers to window object - -
- callback() // "this" inside callback also refers window object -
- } -
- -
- function callback_function(){ -
- -
- // function to be passed as callback -
- -
- // here "THIS" refers to window object also -
- -
- } -
- -
- outer_function(callback_function) -
- // invoke with callback -
--------------------------------------------------------------------------------
Different ways to manipulate this inside callback functions: 在回调函数内部处理this不同方法：
Here I have a constructor function called Person. 在这里，我有一个名为Person的构造函数。 It has a property called name and four method called sayNameVersion1 , sayNameVersion2 , sayNameVersion3 , sayNameVersion4 . 它具有一个名为name的属性和四个名为sayNameVersion1 ， sayNameVersion2 ， sayNameVersion3 ， sayNameVersion4 。 All four of them has one specific task.Accept a callback and invoke it.The callback has a specific task which is to log the name property of an instance of Person constructor function. 它们全部有一个特定的任务。接受一个回调并调用它。回调具有一个特定的任务，即记录Person构造函数实例的name属性。
function Person(name){
this.name = name
this.sayNameVersion1 = function(callback){
callback.bind(this)()
}
this.sayNameVersion2 = function(callback){
callback()
}
this.sayNameVersion3 = function(callback){
callback.call(this)
}
this.sayNameVersion4 = function(callback){
callback.apply(this)
}
}
function niceCallback(){
// function to be used as callback
var parentObject = this
console.log(parentObject)
}
Now let's create an instance from person constructor and invoke different versions of sayNameVersionX ( X refers to 1,2,3,4 ) method with niceCallback to see how many ways we can manipulate the this inside callback to refer to the person instance. 现在，让我们创建一个从人的构造函数和调用不同版本的实例sayNameVersionX （X指的是1,2,3,4）法niceCallback ，看看我们有多少种方法可以操纵this里面回调指person实例。
var p1 = new Person('zami') // create an instance of Person constructor
What bind do is to create a new function with the this keyword set to the provided value. 绑定的作用是使用this关键字设置为提供的值来创建一个新函数。
sayNameVersion1 and sayNameVersion2 use bind to manipulate this of the callback function. sayNameVersion1和sayNameVersion2使用绑定来操纵this回调函数。
this.sayNameVersion1 = function(callback){
callback.bind(this)()
}
this.sayNameVersion2 = function(callback){
callback()
}
first one bind this with callback inside the method itself.And for the second one callback is passed with the object bound to it. 第一个结合this与方法itself.And为第二个回调内部回调被传递与结合于它的对象。
p1.sayNameVersion1(niceCallback) // pass simply the callback and bind happens inside the sayNameVersion1 method
p1.sayNameVersion2(niceCallback.bind(p1)) // uses bind before passing callback
The first argument of the call method is used as this inside the function that is invoked with call attached to it. 的first argument的的call方法被用作this内部时调用与函数call连接到它。
sayNameVersion3 uses call to manipulate the this to refer to the person object that we created, instead of the window object. sayNameVersion3使用call操作this来引用我们创建的人员对象，而不是窗口对象。
this.sayNameVersion3 = function(callback){
callback.call(this)
}
and it is called like the following : 它的名称如下：
p1.sayNameVersion3(niceCallback)
Similar to call , first argument of apply refers to the object that will be indicated by this keyword. 与call相似， apply第一个参数指的是将this关键字指示的对象。
sayNameVersion4 uses apply to manipulate this to refer to person object sayNameVersion4用途apply于操纵this指Person对象
this.sayNameVersion4 = function(callback){
callback.apply(this)
}
and it is called like the following.Simply the callback is passed, 它的调用方式如下所示：只需传递回调，
p1.sayNameVersion4(niceCallback)
