javascript - 从 Javascript 方法访问调用对象

Question

在 JavaScript 中，对于定义在对象内部的匿名函数，this 关键字指的是对象本身。但是，当函数定义在对象范围之外时，this 指的是 window 对象，示例:

function foo() {
  console.log(this)
}

let A = {
  att: foo(),
  att2: "bla"
}

A.foo();

[编辑]:我在原始代码中犯了一个错误，我的意思是将函数调用为 A.att()(我有义务保留 att: foo()。我的代码基本上如下:

function foo() {
  console.log(this)
}

let A = {
  att: foo(),
  att2: "bla"
}

A.att();

这里输出的是对象Window，而不是我要访问的对象A。而且我不能改变A，那么如何从foo()访问A呢？

Answer 1

A function's this keyword behaves a little differently in JavaScript compared to other languages. It also has some differences between strict mode and non-strict mode. In most cases, the value of this is determined by how a function is called. It can't be set by assignment during execution, and it may be different each time the function is called.

要自己操作this 对象，您可以使用JS 方法call、apply 和bind。

让我们用例子来演示每一个:

Function.prototype.call()

详细信息:https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_objects/Function/bind

function foo() {
  console.log(this)
}

var A = {
  att: foo,
  att2: "bla"
}

foo.call(A);

Function.prototype.apply()

详细信息: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply

function foo() {
  console.log(this)
}

var A = {
  att: foo,
  att2: "bla"
}

foo.apply(A);

Function.prototype.bind()

详细信息:https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_objects/Function/bind

function foo() {
  console.log(this)
}

var A = {
  att: foo,
  att2: "bla"
}

var x = foo.bind(A);
x();