为了转生而点技能-JavaScript，day21(let、const介绍

Var：作用域是依照所属的函式位於的区块内，且可以重复宣告。

        for (var i = 0; i < 3; i++) {
        };
        console.log(i);            //3

        (function () {
            for (var j = 0; j < 3; j++) {

            };
        }());
        console.log(j);             //j is not defined
        
        
        var boy = 'TOM';
        var boy = 'BOB';
        console.log(boy);           //BOB

let：作用域是依照所属{}区块内，且不可以重复宣告。

        for (let i = 0; i < 3; i++) {
            console.log(i);                //0 1 2
        };
        console.log(i);                    //i is not defined

        (function () {
            for (var j = 0; j < 3; j++) {

            };
        }());
        console.log(j);             //j is not defined
        
        
        var boy = 'TOM';
        var boy = 'BOB';
        console.log(boy);           //Identifier 'boy' has already been declared
        
        
        {
            let boy = 'TOM';
            console.log(boy);       //TOM
        };
        let boy = 'BOB';
        console.log(boy);           //BOB

TDZ(暂时性死区)：let一样会有hoisting，但是在创造阶段，无法呼叫变数；如果是var，则会出现该变数是undefined的值。

const：作用域是依照所属{}区块内，且不可以重复宣告，或是重复赋值；如果内容(值)是个物件，那麽此物件的内容(物件的参数)是可以更改的。

        var boy = 'TOM';
        boy = 'BOB';
        console.log(boy);     //BOB
        
        
        let boy = 'TOM';
        boy = 'BOB';
        console.log(boy);     //BOB
        
        
        const boy = 'TOM';
        boy = 'BOB';
        console.log(boy);     //Uncaught TypeError: Assignment to constant variable.
        
        
        const family = {
            father: 'TOM',
            mothetr: 'Mar',
        };

        family.father = 'Park'
        console.log(family.father);  //Park
        
        family = {};                //Uncaught TypeError: Assignment to constant variable.