JS function promotion and declaration promotion of VaR variable

Variable Promotion

// The first 1 topic ~~

var a = 1;
function bar() {
    
    console.log(a);
    if (!a) {
    
        var a= 10;
    }
    console.log(a);
}
bar();
// ask ： two console.log() What is the value of .
**// Output ：**
undefined   
10

// analysis ：
Global variable has a, But there is also a function inside a So the global variable a Invalid inside function , because var Declared variables have no block level scope concept therefore if Inside a Promoted to the top of the function And only enhance the declaration part Do not promote the assignment part So the first one console Output the function's internal promotion a But there is no assignment at this point JS Will assign initial values to the variables of life undefined
the second console see if Conditions !a here a yes undefined by false that !a Namely true
Will enter if In scope to a assignment So the second one console Output 10.

There are several concepts to be clear about
var Functional scope But there is no block level scope
Block level scope ： Including loop statement and loop body ; Braces for branch statements ; Or just a pair of braces .
Such as ：
//(1)
if(true){
var a = 1;
}
//(2)
for(var i=0;i<10;i++){
var a = 1;
}
//(3)
{
var a = 1;
}
All of the above will make var Declared variable promotion .

// The first 2 topic ~~

function fun5(a, b) {
    
    var a;
    var b;
    console.log(a, b);
}
fun5(1, 2);

~~ Output
1 2
// analysis ：
It's stated here a,b In fact, the program automatically declares it to us That is to say, the formal parameter variable and the variable we declare inside the function and do not assign a value are one variable ( The premise is that the variable names should be the same )
Formal parameter variables do not write declarations js It will also help us implicitly declare Artificial manual declaration is actually optional , Mainly to confuse us …

// The first 3 topic ~~

function fun() {
    
    var n1 = n2 = n3 = 1;
}
fun();
console.log(n2, n3);

// Output ：
1 1
// analysis ：
Here we need to pay attention to ：
（1） Not inside the function var Variables declared by keywords are treated as global variables , therefore n2 n3 yes Global variables .
var Next to the back n1 Is the declared local variable .
（2） Assignment operator execution order
n3 = 1; // n3 yes 1
n2 = n3; // n2 yes 1
var n1 = n2; // because n1 Is a local variable, so an external call will show undefined .

Function enhancement

c();
var c=2;
function c(){
    
  console.log(5);
}
c();

answer ：

Function enhancement ： Before the program is executed , It will promote the function as a whole to the front of the scope , You can call first and then create .
therefore ： First line c() Yes, you can call function c Of .
Be careful ： The variable name is the same as the function name , Because the function name It is essentially a variable , Saved the function . So the function c It can be written.
var c = function(){
console.log(5);
}
// So the next sentence Namely
var c = 2;
// here c yes 2, It's no longer a method . So the display function is undefined .

a();
function a() {
    
    console.log(111);
}
a();
function a() {
    
    console.log(222);
}
a = function () {
     console.log(333); }
a();

The result is ：222 222 333
explain ： Function expression No promotion ！ Because the equal sign is assignment It's not a statement
~~function The first one will improve

a = function () {
     console.log(333); }
a();
function a() {
    
    console.log(222);
}
a();

function a() Promoted to the front Function expressions do not promote Then be a=function() covers

var a = 5;
function a() {
    
     console.log(111);
 }
console.log('a :>> ', a);

amount to