括号如何影响 JavaScript 中的代码? [英] How do parentheses affect code in JavaScript?

问题描述

var obj, method;

obj = {
  go: function() { console.log(this); }
};

(method = obj.go)()

注意:Fyodor 对他的回答的第一条评论对我帮助最大.正如主题所暗示的那样，这更多是关于括号而不是 this.

NOTE: Fyodor's first comment to his answer is what helped me the most. As the topic suggests, this was more about the parentheses than this.

最后一行，我的理解是括号会强制里面的代码先运行，所以method取的是go属性的值，也就是一个函数.

In the last line, what I understand is that the parentheses will force the code inside to run first, so method takes the value of the go property, which is a function.

() 然后调用该函数，该函数将 window 记录到控制台，因为它不是作为方法调用的.

The () then calls that function, which logs window to the console because it wasn't called as a method.

如果你使用 method = obj.go() 而不是 (method = obj.go)()，它会首先运行 go 函数，method 会取其返回的值.由于 go 什么都没有返回，它将是 undefined.go 打印的值将是 obj.

If instead of (method = obj.go)(), you do method = obj.go(), it will first run the go function, and method will take the value returned by it. Since go returns nothing, it will be undefined. The value printed by go will be obj.

我不明白的是，为什么如果我执行 (obj.go)() 打印的 this 是 obj 而不是窗口?

What I don't understand is, why if I do (obj.go)() the this printed is obj and not window?

考虑到其他代码的工作方式，我预计这段代码的工作方式如下:obj.go 首先在括号内求值，然后函数作为 IIFE (function() { console.log(this); })() 运行.所以由于函数不是作为obj的方法调用的，this默认为window.

Considering how the other code works, I expected this code to work kind of like this: obj.go is evaluated first inside the parentheses and then the function is run as an IIFE (function() { console.log(this); })(). So since the function isn't called as a method of obj, the this defaults to window.

推荐答案

(method = obj.go)() 分两步求值.

1. method = obj.go 被求值并且 method var 变成等于对象 obj 的函数 go代码>.JavaScript 中的函数可以作为方法或函数调用，因此通常它不影响您如何定义函数 go.

1. method = obj.go is evaluated and method var become equal to function go of object obj. Functions in JavaScript can be called as methods or as functions, so in general it doen't metter how you;ve defined function go.

然后 method() 被调用.由于您没有为 this 提供值(通过调用 method 函数作为某个对象的方法或使用 bind 或 call 调用它时 this 设置为全局对象(在非严格模式下)或 undefined(在严格模式下)

Then method() is called. As you didn't provided value for this (either by calling method function as method of some object or using bind or call it is called with this set to global object (in non strict mode) or to undefined (in strict mode)

当你调用 obj.go() this 被设置为等于 obj (它类似于使用 obj.go.call(obj) 或 method.call(obj)).如果你只调用 method() this 等于全局对象(类似于调用 obj.go.call(window))

When you call obj.go() this is set equal to obj (it is similar to use obj.go.call(obj) or method.call(obj)). If you call just method() this is equal to global object (similar to call obj.go.call(window))

编辑

并通过您的示例将 obj 作为 this 获取，您可以这样做

And to get obj as this with your example you can do such way

(method = obj.go.bind(obj))()

通过这种方式，您不仅可以将 go 函数分配给变量 method，还可以创建到特定 this 的绑定等于 obj

In this way you not only assign go function to variable method but create binding to specific this equal to obj

阅读有关 函数调用和 this 的好书JavaScript

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