ES6作用域和解構賦值

ES6 強制開啟嚴格模式

作用域

• var 聲明局部變數，for/if花括弧中定義的變數在花括弧外也可訪問
• let 聲明的變數為塊作用域，變數不可重覆定義
• const 聲明常量，塊作用域，聲明時必須賦值，不可修改

// const聲明的k指向一個對象，k本身不可變，但對象可變

function test() {
    const k={
        a:1
    }
    k.b=3;
    
    console.log(k);
}

test()

解構賦值

{
    let a, b, 3, rest;
    [a, b, c=3]=[1, 2];

    console.log(a, b);
}
//output: 1 2 3

{
    let a, b, 3, rest;
    [a, b, c]=[1, 2];

    console.log(a, b);
}
//output: 1 2 undefined

{
    let a, b, rest;
    [a, b, ...rest] = [1, 2, 3, 4, 5, 6];
    console.log(a, b, rest);
}
//output:1 2 [3, 4, 5, 6]

{
    let a, b;
    ({a, b} = {a:1, b:2})

    console.log(a ,b);
}
//output: 1 2

使用場景

變數交換

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

獲取多個函數值

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

獲取多個函數返回值

{
    function f(){
        return [1, 2, 3, 4, 5]
    }
    let a, b, c;
    [a,,,b] = f();
    console.log(a, b);
}
//output: 1 4

{
    function f(){
        return [1, 2, 3, 4, 5]
    }
    let a, b, c;
    [a, ...b] = f();
    console.log(a, b);
}
//output: 1 [2, 3, 4, 5]

對象解構賦值

{
    let o={p:42, q:true};
    let {p, q, c=5} = o;

    console.log(p ,q);
}
//output: 42 true 5

獲取json值

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