Anonymous function this pointing and variable promotion

Reference article ：https://www.jb51.net/article/161019.htm

var a = 10; 
(function() {
    console.log(a) 
    a = 5 
    console.log(window.a) 
    var a = 20;
    console.log(a)
})()

analysis ： In the immediate function ,var a = 20; Statement defines a local variable a, because js Variable declaration promotion mechanism , local variable a The declaration of is promoted to the top of the body of the function that executes the function immediately , And since such promotion does not include assignment , So the first print statement prints undefined, The last statement will print 20.

Due to variable declaration Promotion ,a = 5; When this statement is executed , Local variables a Already declared , however The execution of anonymous functions is global , Therefore, its effect is on local variables a assignment , here window.a It is still the first assignment 10.

var name = 'window'
var person = {
    name: 'Alan',
    sayOne: function() {
        console.log(this.name)
    },
    sayTwo: function() {
        return function() {
            console.log(this.name)
        }
    }
}
person.sayOne() //Alan
person.sayTwo()() // window

1. intra-function this Point to the caller
2. sayOne The caller is person object , therefore this Point to person;
3. sayTwo Although the caller of is also person object , But the difference is that this call doesn't type this Instead, an anonymous function is returned globally
4. This anonymous function is not called and executed as a method of an object , Is performed globally

Make some changes

var name = 'window'
    var person = {
        name :'Alan',
        sayName:function () {
            var that = this
            return function () {
                console.log(that.name)
            }
        }
    }
    person.sayName()()  // Alan

extend

【 Variable Promotion 】 What is the output of the following code ？

function sayHi() {
    console.log(name);
    console.log(age);
    var name = "Lydia";
    let age = 21;
}
sayHi();

• A: Lydia  and  undefined
• B: Lydia  and  ReferenceError
• C: ReferenceError  and  21
• D: undefined  and  ReferenceError

answer : D

In the function , We use var The keyword declares name Variable . This means that the variable will be promoted during the creation phase （JavaScript Memory space will be allocated to the variable during the creation phase ）, The default value is undefined, Until we actually execute to the line using the variable . We haven't name Variable assignment , So it still remains undefined Value .
Use let keyword （ and const） Declared variables also have variable promotion , But with var Different , Initialization is not promoted . In our statement （ initialization ） Before they , They are inaccessible . This is known as “ A temporary dead zone ”. When we try to access a variable before declaring it ,JavaScript Will throw out a ReferenceError.