var、let、const

	var	let	const
作用域范围	函数	块	块
否覆盖声明	true	err	err
变量提升	true	false	false
再次赋值	true	true	err

javascript对象是引用传递，基本类型是按值传递

ps:javascript里没有引用传递其实是传递引用?

function changeStuff(a, b, c)
{
    a = a * 10;
    b.item = "changed";
    c = {item: "changed"};
}

var num = 10;
var obj1 = {item: "unchanged"};
var obj2 = {item: "unchanged"};

changeStuff(num, obj1, obj2);

console.log(num);
console.log(obj1.item);
console.log(obj2.item);

console.log([1] === [1])//false
console.log([1] == [1])//false

• json 对象的拷贝函数

function copy(o) {
    if (Object.prototype.toString.call(o) === '[object Array]') {

        return o.map(item=>copy(item));

    } else if (Object.prototype.toString.call(o) === '[object Object]') {

        return Object.keys(o).reduce((result, key)=> {
            result[key] = copy(o[key]);
            return result;
        }, {})

    }
    else {
        return o;
    }
}