Problems related to self calling functions and automatically defining var because variables are not declared

First, give the default undeclared variables var Theorem ：

Function body , Do not declare variables （ Such as a=2） He will look up the definition of this variable from his superior , When you find the top layer , If not defined , It will be used automatically var Definition （ Non strict mode ）

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And under the self calling function It has and not only has the following characteristics ：

1. Function declaration and execution are continuous Together. , Both No function declaration promotion
2. From within the calling function The definition of a function is Constant

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Go straight to the code Use topics to consolidate

For self calling functions

// Function case 1 
var a=1;
var a=2;
(function a() {
    console.log(a);// Output a Function content   Because there is no function declaration promotion 
})()
console.log(a);//2 a Functions are declared and defined in self calling functions   It's not going out   and var Repeat statement 2 covers 1

// Function case 2
var a=1;
(function a() {
    a=2; // Because there are a function   Inconformity theorem   Can't var  But in the self calling function a Is a constant   Constants cannot be modified   here a=2 Is an invalid operation   Report in strict mode const error 
    console.log(a);// Output a Function content 
})()
console.log(a);//1  There is no trigger theorem   No, var  So from the calling function a Your statement has not been circulated 

// Function case 3
(function b() {
    a=2; // There is no a  Coincidence theorem   therefore var a=2 
    console.log(a);// Output 2
})()
console.log(a);//2  Coincidence theorem   therefore var a=2  Passed outside the calling function 

Function case 1 in , Because the function declaration is not promoted , So it won't show up a=2 hold a Replace function Lead to “ primary a function ” Can't be called .

Function case 2 in , Because there are a function Inconformity theorem Can't var But in the calling function function a Is a constant Constants cannot be modified here a=2 Is an invalid operation Report in strict mode const error

Function case 3 in , It conforms to the theorem , So it's equivalent to the outermost layer There is one var a=2 So it comes out of the function body

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For ordinary functions

// Function case 4   Under normal circumstances 
function a(){
   var a=2 //a=2 Is defined in the function body   It's not going out 
   console.log(a)//2
}
a()
console.log(a)//a Function content 

// Function case 5 
function a(){
   a=2// Compare the function 2  There are a function   Inconformity theorem   Can't var a Function is not constant a=2 Not defined in a function   therefore a=2 covers a function   So the effect is almost equal to the function case 6
   console.log(a)//2
}
a()
console.log(a)//2

// Function case 6
function a(){}
a=2// here  a The value is replaced with 2
console.log(a)//2
console.log(a)//2

It's mainly here Function case 5 To compare , contrast Function case 2, There are a function Inconformity theorem Can't var,a Function is not constant And a=2 Not defined in a function therefore a=2 Find the outer layer defined a Function and covers a function So the effect is almost equal to Function case 6