SOURCE

// {
//     let a = 1
//     var b = 2
// }
// console.log(b)
// console.log(a)
// let a
//暂时性死区
// let tmp =123
// if(true){
//     console.log(tmp)
//     typeof tmp //意味着typeof不再是百分百安全的操作
//     let tmp = 222
//     console.log(tmp)
// }


//不容许重复声明
// {
//     let a =1
//     var a =2
// }


//常量必须要赋值
const abc = null

//块级作用域内层不影响外层

{
    let a = 123
    {
        let a = 234
    }
    //console.log(a)
}

//块级作用域的出现，使得立即执行函数不在必须

//IIFE写法
(function() {

})()
//块级作用域写法
{

}


if(true) {
    function fn() {console.log('fn提升')}
}
try{
    function fn1() {console.log('fn1提升')}
}catch(e) {

}
// fn()
// fn1()


//函数在块级作用域中声明,在块级作用域外不可引用
function f() { console.log('I am outside!'); }

(function () {
  if (false) {
    // 重复声明一次函数f
    function f() { console.log('I am inside!'); }
  }

  //f();
}());
// 等同于
function f() { console.log('I am outside!'); }

(function () {
    var f= undefined
  if (false) {
    // 重复声明一次函数f
    function f() { console.log('I am inside!'); }
  }
  //f();
}());

{
  let a = 'secret';
  function f() {
    return a;
  }
  //console.log(f())
}


//提案：块级作用域的返回值
// let x = do {
//     let x = 123
//     return x
// }


//彻底冻结对象方法

function freezeM(obj) {
    Object.freeze(obj)
    //
    Object.keys(obj).forEach((key)=> {
        if(typeof obj[key] === 'object') {
            freezeM(obj[key])
        }
    })
}


//顶层对象属性
gg =1 //window.gg = 1
function fgg(){} //window.fgg = function fgg(){}
//console.log(gg)

let g =123// let const class import
//console.log(window.g) //undefined 

function log(...args) {
    return console.log(...args)
}

// 变量的结构与赋值

let [a,b] = [1,2]
//log(a,b)


let [x,y,z] = new Set([1,2,3])


//Generator 写一个斐波那契额数列
function *ff() {
    let x =0
    let y =1
    while(true) {
        //log(y)
        yield y
        x = y
        y = x+y
    }
}
let gen = ff()
for(let i=0;i<10;i++) {
   gen.next()
}

//
const {a:bbb=333} = {a:undefined}
//log(bbb)


//
let arrayList = [1,2,3,4,5]
const {0:start,[arrayList.length-1]:end} = arrayList
// log(start)
// log(end)

//
let xxx
({xxx} = {xxx:1})
//log(xxx)


//'length'.forEach((s)=> {log(s)}) //"length".forEach is not a function
Array.prototype.forEach.call('length',(s)=> {
    //log(s)
})


var words = ['one', 'two', 'three', 'four'];
words.forEach(function(word) {
  //console.log(word);
  if (word === 'two') {
    words.shift();
  }
});