ES6中const声明的完整机制与内存管理解析

const a = 5
console.log(a)
console.log(window.a)
// 5
// undefined

cosnt a = 5
cosnt a = 6
// Uncaught SyntaxError: Unexpected identifier 'a'
不能重复声明a

if(true) {
    console.log(a)
    const a = 5
}
// Uncaught ReferenceError: Cannot access 'a' before initialization
// 初始化前无法访问a

if (true) {
	const a = 5
}
console.log(a)
// Uncaught ReferenceError: a is not defined
// a 再大括号外没有声明

// 定义一个新的属性
Object.defineProperty(window, 'PI', {
	value: 3.14,
	writable: false, // 是否可修改的, false表示不可以修改
})
console.log(PI) // 3.14
PI = 5
console.log(PI) // 3.14 ，因为不可以被修改

const a = 5;
a = 6;
// Uncaught TypeError: Assignment to constant variable.
// 不能修改常量的值

const a;
a = 5
//  Uncaught SyntaxError: Missing initializer in const declaration
// 常量必须再声明时赋值

const obj = {
	name: 'xiaoxiao',
	age: 18
}
console.log(obj)
// {name: 'xiaoxiao', age: 18}
obj.school = 'imooc'
console.log(obj)
// {name: 'xiaoxiao', age: 18, school: 'imooc'}

const arr = [1, 2, 3]
arr.push(4)
console.log(arr)
// [1,2,3,4]

const obj = {
	name: 'xiaoxiao',
	age: 18
}
Object.freeze(obj)
console.log(obj)
// {name: 'xiaoxiao', age: 18}
obj.school = 'imooc'
console.log(obj)
// {name: 'xiaoxiao', age: 18}

const obj = {
	name: 'xiaoxiao',
	age: 18,
    sikll: {
        name: 'code',
        year: 11
    }
}
Object.freeze(obj)
obj.school = 'imooc'
obj.sikll.year = 13
console.log(obj)
/*
{
    "name": "xiaoxiao",
    "age": 18,
    "sikll": {
        "name": "code",
        "year": 13
    }
}
*/

const obj = {
	name: 'xiaoxiao',
	age: 18,
    sikll: {
        name: 'code',
        year: 11
    }
}
Object.freeze(obj) // 冻结第一层属性，不可以被修改
Object.freeze(obj.sikll) // 冻结第二层属性，不可以被修改
obj.school = 'imooc'
obj.sikll.year = 13
console.log(obj)
/*
{
    "name": "xiaoxiao",
    "age": 18,
    "sikll": {
        "name": "code",
        "year": 11
    }
}
*/

const arr = [1,2,3]
Object.freeze(arr)
arr.push(4)
console.log(arr)
// Uncaught TypeError: Cannot add property 3, object is not extensible
// 无法添加属性 3，对象不可扩展