匿名函数的上下文是什么？

发布于 2024-11-26 10:38:43 字数 650 浏览 1 评论 0原文

我有这样的代码：

function demo() {
    this.val=5;
    function() {
        this.val=7;
    }();
}

现在，当我在 Firefox 或 Chrome 控制台中执行此代码时，它会出现语法错误。我不明白为什么这是一个错误，因为我已经读到 javascript 函数是对象，因此当我调用匿名函数时，其中的 this 指向函数 demo 并且应该更改 val< /code> 到 7，所以如果我这样做

var x=new demo();
x.val;   //should give 7

，但是当我这样做时

function demo() {
    this.val=5;
    var f=function() {
            this.val=7;
    }();
}
window.val;    // gives 7

，我不明白函数是否是对象，那么为什么匿名函数中的 this 指向 窗口而不是演示。请解释一下。

原文

I have code like this:

function demo() {
    this.val=5;
    function() {
        this.val=7;
    }();
}

Now when I give execute this code in the firefox or chrome console it gives a syntax error. I don't understand why this is an error because I have read that javascript functions are objects so when I call the anonymous function, inside it this points to function demo and should change the val to 7, so if I do

var x=new demo();
x.val;   //should give 7

but when I do this

function demo() {
    this.val=5;
    var f=function() {
            this.val=7;
    }();
}
window.val;    // gives 7

I don't understand if functions are objects then why the this in the anonymous function points to window and not demo.
Please explain this.

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原野 2024-12-03 10:38:43

我不明白为什么这是一个错误

，因为

function() {
    this.val=7;
}();

被评估为函数声明并且函数声明需要一个名称。要使其解释为函数表达式，您需要将其放在括号中：（

 (function() {
    this.val=7;
 }());

使其成为立即函数）

或将函数分配给变量：

var f = function() {....};
f();

您在第二个片段中所做的事情是两者的混合虽然有效，但没有多大意义。 f 的值为 undefined。

我不明白函数是否是对象，那么为什么匿名函数中的 this 指向 window 而不是 demo。

函数不会继承调用它们的上下文。每个函数都有自己的 this，而 this 所指的内容取决于该函数的方式被称为。它基本上可以归结为：

“独立”（func()）或作为立即函数（(function(){...}())）： this 将引用全局对象（浏览器中的window）
作为对象的属性 (obj.func())：this 将引用 obj
使用 new ^[docs] 关键字 ( new Func() )：this 指的是一个空对象，继承自Func.prototype
应用 < em>^[文档] 和 <代码>调用 ^[docs] 方法（ func.apply(someObj) ）：this 引用 someObj

进一步阅读：

i dont understand why this is an error

because

function() {
    this.val=7;
}();

is evaluated as function declaration and function declarations need a name. To make it interpreted as function expression, you need to put it in parenthesis:

 (function() {
    this.val=7;
 }());

(making it an immediate function)

or assign the function to a variable:

var f = function() {....};
f();

What you do in the second snippet is a mixture of both and although being valid, it does not make much sense. f will have the value undefined.

i dont understand if functions are objects then why the this in the anonymous function points to window and not demo.

Functions don't inherit the context they are called in. Every function has it's own this and what this refers to is determined by how the function is called. It basically boils down to:

"Standalone" ( func()) or as immediate function ( (function(){...}()) ) : this will refer to the global object (window in browsers)
As property of an object ( obj.func()): this will refer to obj
With the new ^[docs] keyword ( new Func() ): this refers to an empty object that inherits from Func.prototype
apply ^[docs] and call ^[docs] methods ( func.apply(someObj) ): this refers to someObj