var a = 100;
console.log(a,window.a); // 100 100
let b = 10;
console.log(b,window.b); // 10 undefined
const c = 1;
console.log(c,window.c); // 1 undefined
console.log(a); // undefined ===> a已声明还没赋值,默认获得undefined值
var a = 100;
console.log(b); // 报错:b is not defined ===> 找不到b这个变量
let b = 10;
console.log(c); // 报错:c is not defined ===> 找不到c这个变量
const c = 10;
if(1){
var a = 100;
let b = 10;
}
console.log(a); // 100
console.log(b) // 报错:b is not defined ===> 找不到b这个变量
if(1){
var a = 100;
const c = 1;
}
console.log(a); // 100
console.log(c) // 报错:c is not defined ===> 找不到c这个变量
var a = 100;
console.log(a); // 100
var a = 10;
console.log(a); // 10
let a = 100;
let a = 10;
// 控制台报错:Identifier 'a' has already been declared ===> 标识符a已经被声明了。
var a = 100;
if(1){
a = 10;
//在当前块做用域中存在a使用let/const声明的状况下,给a赋值10时,只会在当前做用域找变量a,
// 而这时,还未到声明时候,因此控制台Error:a is not defined
let a = 1;
}
/*
* 一、一旦声明必须赋值,不能使用null占位。
*
* 二、声明后不能再修改
*
* 三、若是声明的是复合类型数据,能够修改其属性** */const a = 100; const list = [];list[0] = 10;console.log(list); // [10]const obj = {a:100};obj.name = 'apple';obj.a = 10000;console.log(obj); // {a:10000,name:'apple'}