ES6（解构赋值）

解构赋值，分为左右两种解构

• 数组解构赋值 左右两边都是数组
• 对象解构赋值 左右两边都是对象
• 字符串解构赋值 左边是数组，右边是字符串
• 布尔解构赋值（就是对象解构赋值）
• 函数解构赋值
• 数值解构赋值（就是对象解构赋值）

//数组解构赋值
{
    //用let 声明三个变量
    let a, b, rest;
    //相当于a=1 b=2
    [a, b] = [1, 2];
    console.log(a, b);// 1 2
}

//数组解构赋值
{
    let a, b, rest;
    [a, b, ...rest] = [1, 2, 3, 4, 5, 6];
    console.log(a, b, rest);// 1 2 [3,4,5,6]
}

//对象解构赋值
{
    let a, b;
    ({a, b} = {a: 1, b: 2});
    console.log(a, b);// 1 2
}

//数组解构赋值 默认值
{
    let a, b, c, rest;
    [a, b, c = 3] = [1, 2];
    console.log(a, b, c);// 1 2 3
}

//数组解构赋值 若没有在解构上成功配对，那么变量相当于只声明未赋值
{
    let a, b, c, rest;
    [a, b, c] = [1, 2];
    console.log(a, b, c);// 1 2 undefined
}

//利用解构赋值 实现变量交换
{
    let a = 1;
    let b = 2;
    [a, b] = [b, a];
    console.log(a, b);//2 1
}

{
    function f() {
        return [1, 2];
    }

    let a, b;
    [a, b] = f();
    console.log(a, b);//1 2
}

{
    function f1() {
        return [1, 2, 3, 4, 5];
    }

    let a, b, c;
    [a, , , b] = f1();
    console.log(a, b);//1 4
}

{
    function f2() {
        return [1, 2, 3, 4, 5];
    }

    let a, b, c;
    [a, ...b] = f2();
    console.log(a, b);//1 [2,3,4,5]
}

//对象解构赋值
{
    let o = {p: 42, q: true};
    let {p, q} = o;
    console.log(p, q);//42 true
}
//对象解构赋值 默认值
{
    let {a = 10, b = 5} = {a: 3};
    //a默认值为10 b默认值为5，通过赋值，a的值被覆盖为3，b没有被覆盖
    console.log(a, b);//3 5
}

//对象解构赋值 嵌套应用
{
    let metaData = {
        title: 'abc',
        test: [{
            title: 'test',
            desc: 'description'
        }]
    };

    let {title: esTitle, test: [{title: cnTitle}]} = metaData
    console.log(esTitle, cnTitle);//abc test
}