An article explaining the problem of this pointing in js (with code)
In the previous article "Mid-Autumn Festival Tips: How to Use CSS to Realize the Revolution of the Earth and the Moon (Collection)", I introduced you to how to use CSS to realize the rotation of the Earth and the Moon. The following article will help you understand the this pointing problem in js. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.
This points to the problem in JS
Believe me, as long as you remember the 7 steps in this article, you can completely master the ## in JS #this points to.
this points to it.
Console console of Chrome.
this will not be changed, as long as the current function is an arrow function, there is no need to look at other rules.
this of an arrow function is the point of the outer this when it is created. There are two key points here:
this direction has been determined.
this in the arrow function points to this in the outer layer.
this of the arrow function, you must first know the direction of the outer layer this, and you need to continue to apply the seven-step formula in the outer layer.
When using the new keyword to call a function, this in the function must be a new object created by JS.
Readers may have questions, "If thenew key is used to call the arrow function, will this of the arrow function be modified?".
func = () => {}
new func() // throw error
new.
bind refers to Function.prototype.bind() Detailed address: https://developer.mozilla.org/zh-CN/docs/Web/JavaScript /Reference/Global_Objects/Function/bindWhen binding multiple times, only the value of the first bind is recognizedError-prone points
function func() {
console.log(this)
}
func.bind(1).bind(2)() // 1func = () => {
// 这里 this 指向取决于外层 this，参考口诀 7 「不在函数里」
console.log(this)
}
func.bind(1)() // Window，口诀 1 优先Copy after login
bind and newError-prone pointsfunc = () => {
// 这里 this 指向取决于外层 this，参考口诀 7 「不在函数里」
console.log(this)
}
func.bind(1)() // Window，口诀 1 优先function func() {
console.log(this, this.__proto__ === func.prototype)
}
boundFunc = func.bind(1)
new boundFunc() // Object true，口诀 2 优先apply()and The first parameter of call() is both this. The difference is that when calling through apply, the actual parameters are placed in the array, while through callThe actual parameters are separated by commas when called.
func = () => {
// 这里 this 指向取决于外层 this，参考口诀 7 「不在函数里」
console.log(this)
}
func.apply(1) // Window，口诀 1 优先
function func() {
console.log(this)
}
boundFunc = func.bind(1)
boundFunc.apply(2) // 1，口诀 3 优先function func() {
console.log(this.x)
}
obj = { x: 1 }
obj.func = func
obj.func() // 1Copy after login
There is no need for code examples here. Arrow functions and bind functions have higher priority. If you are interested, you can try it yourself. 6. Direct call When the function does not meet the previous scenario and is called directly, function func() {
console.log(this.x)
}
obj = { x: 1 }
obj.func = func
obj.func() // 1this will point to the global object. The global object is Window in the browser environment and Global in the Node.js environment.
function func() {
console.log(this)
}
func() // WindowouterFunc serves the purpose of confusion.
function outerFunc() {
console.log(this) // { x: 1 }
function func() {
console.log(this) // Window
}
func()
}
outerFunc.bind({ x: 1 })() tag of the browser, or# In the module file of ##Node.js. 1. In the
tag, this points to Window. 2. In the module file of
, this points to the default export object of Module, which is module. exports. Non-strict mode
Strict mode was proposed in
ES5. Before the ES5 specification, that is, in non-strict mode, this cannot be undefined or null. So **In non-strict mode, through the above seven steps, if this points to undefined or null, then this will point to the global object. **The global object is Window in the browser environment and Global in the Node.js environment. For example, in the following code, in non-strict mode,
all points to the global object. <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>function a() {
console.log("function a:", this)
;(() => {
console.log("arrow function: ", this)
})()
}
a()
a.bind(null)()
a.bind(undefined)()
a.bind().bind(2)()
a.apply()</pre><div class="contentsignin">Copy after login</div></div>The execution result in non-strict mode is:
在严格模式下，执行同样的代码进行对比。记住要一次性将所有代码复制粘贴到控制台中，才能运行在严格模式下（因为第一行 "use strict" 才会对后面的代码生效）。
"use strict"
function a() {
console.log("function a:", this)
;(() => {
console.log("arrow function: ", this)
})()
}
a()
a.bind(null)()
a.bind(undefined)()
a.bind().bind(2)()
a.apply()严格模式下执行结果为：
七步口诀在严格模式下和非严格模式下都是完备的，只是在非严格模式下null或undefined会被转换为全局对象。所以我没有将这点列入口诀中。
做题复习
先背诵口诀再做题，“箭头函数、new、bind、apply和call、欧比届点（obj.）、直接调用、不在函数里”。
1. 下面代码执行后，func.count 值为多少？
function func(num) {
this.count++
}
func.count = 0
func(1)答案
func.count值为 0。
按照口诀，func()调用时属于第 6 类「直接调用」。在非严格模式下，this指向全局对象。this跟func 一点关系都没有，所以 func.count保持不变so easy。
2. 以下箭头函数中 this 指向谁呢？
obj = {
func() {
const arrowFunc = () => {
console.log(this._name)
}
return arrowFunc
},
_name: "obj",
}
obj.func()()
func = obj.func
func()()
obj.func.bind({ _name: "newObj" })()()
obj.func.bind()()()
obj.func.bind({ _name: "bindObj" }).apply({ _name: "applyObj" })()答案
// obj // undefined // newObj // undefined // bindObj
是不是很简单，你学废了吗？
推荐学习：JS视频教程
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