JavaScript中的变量声明你知道吗

var a；//仅定义
var b=‘hi'；//定义的同时，设置值

var a=”hello”
a=”hi”

Var a=“hello”，
    b=12，
    c=false；

function test(){
    var a="shanxi";
}
test();//调用函数
console.log(a);
//ReferenceError: a is not defined
//报错：a未定义

function test(){
    a="sichuan";
}
test();//调用函数
console.log(a);
//sichuan

function test(){
   console.log(a)
   var a=12;
}
test()
//undefined

function test(){
   var a; 
   console.log(a)
   a=12;
}
test()
//undefined

if(true){
    var b="zhang"
    console.log(b);//zhang
}
console.log(b);//zhang

if(true){
    let b="zhang"
    console.log(b);//zhang
}
console.log(b);//ReferenceError: b is not defined

var a=24;var a=12;var a=5;console.log(a);//5let b=2;let b=4;//SyntaxError: Identifier 'b' has already been declared//运行到此处就已经报错let b=8;console.log(b);var a=24;
var a=12;
var a=5;
console.log(a);//5
let b=2;
let b=4;//SyntaxError: Identifier 'b' has already been declared
//运行到此处就已经报错
let b=8;
console.log(b);

var a="shanxi"
console.log(a);//shanxi
if(true){
    var a="sichaun";
    console.log(a);//sichuan
}
let b=3;
console.log(b);//3
if(true){
    let b=5;
    console.log(b);//5
}

var a;
let a;//SyntaxError: Identifier 'a' has already been declared
let c;
var c;//SyntaxError: Identifier 'c' has already been declared

console.log(a);
var a=2;//undefined
console.log(b);
let b=3;//ReferenceError: Cannot access 'b' before initialization

var a=3;
console.log(window.a)//a
let b=10;
console.log(window.b);//undefined

const n="zhang";
n="li";//TypeError: Assignment to constant variable.
const a=12;
const  a=9;//不可以重复声明
//SyntaxError: Identifier 'a' has already been declared