ECMAScript 6 变量解构赋值

为什么解构很有用

ECMAScript 5以及以前的版本:

let options = {
  repeat: true,
  save: false
};
// extract data from the object
let repeat = options.repeat, 
    save = options.save;

虽然这段代码看上去也挺简单的,但想象一下如果你要给大量的变量赋值,你得一个一个的赋值。
或者你需要取一个嵌套结构数据的某个值,也许你得遍历整个结构。
如果你能把数据解构成一些小小的片段,那获取信息将会更加容易。

对象的解构

let node = {
  type: "Identifier",
  name: "foo"
};
let { type, name } = node;
console.log(type);      // "Identifier"
console.log(name);      // "foo"

注意: 必须初始化

// syntax error!
var { type, name };
// syntax error!
let { type, name };
// syntax error!
const { type, name };

解构赋值

可以赋值给已经定义过的变量:

let node = {
    type: "Identifier",
    name: "foo"
},
type = "Literal",
name = 5;
// assign different values using destructuring
({ type, name } = node);
console.log(type);      // "Identifier"
console.log(name);      // "foo"

默认值

let node = {
    type: "Identifier",
    name: "foo"
};
let { type, name, value = true } = node;
console.log(type); // "Identifier"
console.log(name); // "foo"
console.log(value); // true

给不同名本地变量赋值

let node = {
    type: "Identifier",
    name: "foo"
};
let { type: localType, name: localName } = node;
console.log(localType);     // "Identifier"
console.log(localName);     // "foo"

嵌套对象解构

let node = {
    type: "Identifier",
    name: "foo",
    loc: {
        start: {
            line: 1,
            column: 1 
            },
        end: {
            line: 1,
            column: 4
            }
        } 
};
let { loc: { start }} = node;
console.log(start.line); // 1
console.log(start.column); // 1

数组的解构

let colors = [ "red", "green", "blue" ];
let [ firstColor, secondColor ] = colors;
console.log(firstColor);        // "red"
console.log(secondColor);       // "green"

只取你需要的部分

let colors = [ "red", "green", "blue" ];
let [ , , thirdColor ] = colors;
console.log(thirdColor);        // "blue"

注意: 和对象的解构一样,必须初始化

解构赋值

可以赋值给已经定义过的变量:

let colors = [ "red", "green", "blue" ],
    firstColor = "black",
    secondColor = "purple";
[ firstColor, secondColor ] = colors;
console.log(firstColor);        // "red"
console.log(secondColor);       // "green"

在ECMAScript 5 中交换变量值

let a = 1,
    b = 2, tmp;
tmp = a;
a = b;
b = tmp;
console.log(a); // 2
console.log(b); // 1

在ECMAScript 6 中交换变量值

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

默认值

let colors = [ "red" ];
let [ firstColor, secondColor = "green" ] = colors;
console.log(firstColor);        // "red"
console.log(secondColor);       // "green"

嵌套数组解构

let colors = [ "red", [ "green", "lightgreen" ], "blue" ];
// later
let [ firstColor, [ secondColor ] ] = colors;
console.log(firstColor);        // "red"
console.log(secondColor);       // "green"

剩余的元素

let colors = [ "red", "green", "blue" ];
let [ firstColor, ...restColors ] = colors;
console.log(firstColor);        // "red"
console.log(restColors.length); // 2
console.log(restColors[0]);     // "green"
console.log(restColors[1]);     // "blue"

数组的第一个值赋给了firstColor,剩下的值组成了一个新的数组赋给了restColors。

ECMAScript 5克隆一个数组:

var colors = [ "red", "green", "blue" ];
var clonedColors = colors.concat();
console.log(clonedColors);   // "[red,green,blue]"

ECMAScript 6克隆一个数组:

let colors = [ "red", "green", "blue" ];
let [ ...clonedColors ] = colors;
console.log(clonedColors);  // "[red,green,blue]"

注意: 剩余的元素必须是解构数组的最后一个元素,后面不能有逗号。

混合解构

对象与数组嵌套混合的解构:

let node = {
    type: "Identifier",
    name: "foo",
    loc: {
        start: {
            line: 1,
column: 1 },
        end: {
            line: 1,
column: 4 }
},
    range: [0, 3]
};
let {
    loc: { start },
    range: [ startIndex ]
} = node;
console.log(start.line); // 1
console.log(start.column); // 1
console.log(startIndex); // 0

参数解构

function setCookie(name, value, { secure, path, domain, expires }) {
    // code to set the cookie
}
setCookie("type", "js", {
    secure: true,
    expires: 60000
});

解构的参数是必需的

// error!
setCookie("type", "js");

它实际上是这样运行的:

function setCookie(name, value, options) {
    let { secure, path, domain, expires } = options;
    // code to set the cookie
}

当解构赋值的右边是null或者undefined,就会抛出错误。

如果你希望解构参数是可选的,你可以这样写:

function setCookie(name, value, { secure, path, domain, expires } = {}) {
    // empty }

解构参数的默认值

function setCookie(name, value,
    {
        secure = false,
        path = "/",
        domain = "example.com",
        expires = new Date(Date.now() + 360000000)
} = {} ){
// empty }