ES6 随记（2）-- 解构赋值

上一章请见：

3. 解构赋值

a. 数组的解构赋值

let [a1, b1, c1] = [1, 2, 3];
console.log(a1, b1, c1);        // 1 2 3

let [a2, , c2] = [1, 2, 3];
console.log(a2, c2);            // 1 3

let [a3, b3, c3] = [1, 2];
console.log(a3, b3, c3);       // 1 2 undefined

let [a4, b4, ...c4] = [1, 2]; // 只有最后一位可以用 ...
console.log(a4, b4, c4);      // 1 2 []

let [a5, b5, c5, ...d5] = [1, 2];
console.log(a5, b5, c5, d5);  // 1 2 undefined []

let [a6, [b6], c6] = [1, [2, 3], 3];
console.log(a6, b6, c6);     // 1 2 3

另一个是默认值的概念

let [a1 = 1] = [];
console.log(a1);  // 1
let [a2 = 1] = [2];
console.log(a2);  // 2

let [a3 = 1, b3 = 1, c3 = 1, d3 = 1] = [undefined, null, [], 0];
console.log(a3, b3, c3, d3);  // 1 null [] 0

function f() {
  console.log('aaa');
  return 'a';
}
// 被赋值时不执行 fn
let [a4 = f()] = ['x'];
console.log(a4);  // x
let [b4 = f()] = [];
console.log(b4);  // aaa a

let [a51 = 1, b51 = a51] = [];
console.log(a51, b51);  // 1 1
let [a52 = 3, b52 = a52] = [1, 2];
console.log(a52, b52);  // 1, 2
let [a53 = b53, b53 = 1] = [];
console.log(a53, b53);  // 报错，因为 b53 还未定义不能赋给 a53

b. 对象的解构赋值

当变量只有键时，选择赋值方相同键对应的值给该键；当变量为键值对时，选择赋值方相同键对应的值给该值。

let { a1, b1 } = { a1: "aaa", b1: "bbb" };
console.log(a1, b1);  // aaa bbb
let { c1 } = { x: "aaa", y: "bbb" };
console.log(c1); // undefined

let { x: a2, y: b2 } = { x: "aaa", y: "bbb" };
console.log(a2, b2);  // aaa bbb
let c2; ({c2} = {c2: 'aaa'}); // 必须加括号
console.log(c2);    // aaa

let obj = {
    p: [
        'Hello',
        { b3: 'World' }
    ]
};
let { p: [a3, { b3 }] } = obj;
console.log(a3, b3);  // Hello World

let obj2 = {}; let arr = [];
({ x: obj2.xx, y: arr[0]} = { x: 123, y: true });
console.log(obj2, arr);  // {xx: 123} [true]

let { PI, sin, cos } = Math;
console.log(PI, sin, cos); // 3.141592653589793 function sin() { [native code] } function cos() { [native code] }

let arr2 = [1, 2, 3];
let {0 : first, [arr2.length - 1] : last} = arr2;
console.log(first, last);  // 1 3

对象的解构赋值也可以设置默认值，与数组的解构赋值基本类似

c. 其他解构赋值

let [a, b, c, d, e] = 'hello';
console.log(a, b, c);  // h e l

let {length : len} = 'hello';
console.log(len);  // 5

let {toString: s} = true;
console.log(s === Boolean.prototype.toString) // true

d. 函数参数关于解构赋值的运用

function add([x, y]){
    // 即 let [x, y] = [1, 2]
    return x + y;
}
add([1, 2]); // 3

let x = [[1, 2], [3, 4]].map(([a, b]) => a + b);
console.log(x);  // [3, 7]

function xx(...values) {
    console.log(values);
}
xx(1, 2, 3);  // [1, 2, 3]

// 对比下面两段代码，请注意参数的默认值问题
// -------------
function fn1({x = 0, y = 0} = {}) {
    // 即：先合并 {x=0, y=0} 与 {x: 3, y: 8} 再看是否需要默认值
    return [x, y];
}
fn1({x: 3, y: 8}); // [3, 8]
fn1({x: 3}); // [3, 0]
fn1({}); // [0, 0]
fn1(); // [0, 0]

function fn2({x, y} = { x: 0, y: 0 }) {
    return [x, y];
}
fn2({x: 3, y: 8}); // [3, 8]
fn2({x: 3}); // [3, undefined]
fn2({}); // [undefined, undefined]
fn2(); // [0, 0]

e. 解构赋值的其他应用

// 交换变量的值
let x = 1, y = 2;
[x, y] = [y, x];
console.log(x, y); // 2 1

// 无序对应
function f({x, y, z}) {
    console.log(x, y, z); // 1 2 3
}
f({z: 3, y: 2, x: 1});

// 拿到 json 对应的值
let jsonData = {
    id: 42,
    status: "OK",
    data: [867, 5309]
};
let { id, status, data: number } = jsonData;
console.log(id, status, number);  // 42 OK [867, 5309]

// 设置参数的默认值
function guid(len=5) {
    return Math.random().toString(36).substring(2, len+2);
}

// 与 for-of 愉快地玩耍
let data = [
    {Id: 1, Name: 'zyh', Flag: false},
    {Id: 2, Name: 'zp'},
]
for (let {Name: name} of data) {
    console.log(name);  // 分别打印 zyh, zp
}

// 与模块愉快地玩耍
const { fn1, fn2 } = require("utils");

本文部分转载自阮一峰的 ECMAScript 6 入门