深入理解ES6之—数据解构

一 对象解构

对象解构语法在赋值语句的左侧使用了对象字面量

let node = {
    type: true,
    name: false
}

//既声明又赋值
let {
    type,
    name
} = node;

//或者先声明再赋值
let type, name
({type,name} = node);
console.log(type);//true
console.log(name);//false

type与name标识符既声明了本地变量，也读取了对象的相应属性值。

解构赋值表达式的值为表达式右侧的值。当解构表达式的右侧的计算结果为null或者undefined时，会抛出错误。

默认值

当你使用解构赋值语句时，如果所指定的本地变量在对象中没有找到同名属性，那么该变量会被赋值为undefined

let node = {
    type: true,
    name: false
},
    type, name, value;
({type,value,name} = node);

console.log(type);//true
console.log(name);//false
console.log(value);//undefined

你可以选择性地定义一个默认值，以便在指定属性不存在时使用该值。

let node = {
        type: true,
        name: false
    },
    type, name, value;
({
    type,
    value = true,
    name
} = node);

console.log(type);//true
console.log(name);//false
console.log(value);//true

赋值给不同的本地变量名

let node = {
    type: true,
    name: false,
    value: "dd"
}
let {
    type: localType,
    name: localName,
    value: localValue = "cc"
} = node;
console.log(localType);
console.log(localName);
console.log(localValue);

type:localType这种语法表示要读取名为type的属性，并把它的值存储在变量localType上。该语法与传统对象字面量的语法相反

嵌套的对象结构

let node = {
type: "Identifier",
name: "foo",
loc: {
    start: {
        line: 1,
        column: 1
    },
    end: {
        line: 1,
        column: 4
    }
}
}

let {
loc: localL,
loc: {
    start: localS,
    end: localE
}
} = node;

console.log(localL);// start: {line: 1,column: 1},end: {line: 1,column: 4}
console.log(localS);//{line: 1,column: 1}
console.log(localE);//{line: 1,column: 4}

当冒号右侧存在花括号时，表示目标被嵌套在对象的更深一层中（loc: {start: localS,end: localE}）

二 数据解构

数组解构的语法看起来跟对象解构非常相似，只是将对象字面量换成了数组字面量。

let colors = ["red", "blue", "green"];
let [firstC, secondC, thirdC, thursC = "yellow"] = colors;
console.log(firstC//red
console.log(secondC);//blue
console.log(thirdC);//green
console.log(thursC);//yellow

你也可以在解构模式中忽略一些项，并只给感兴趣的项提供变量名。

let colors = ["red","green","blue"];

let [,,thirdC] = colors;
console.log(thirdC);//blue

thirdC之前的逗号是为数组前面的项提供的占位符。使用这种方法，你就可以轻易从数组任意位置取出值，而无需给其他项提供名称。

解构赋值

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

数组解构有一个非常独特的用例，能轻易的互换两个变量的值。

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

嵌套的解构

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

剩余项

let colors = ["red", "green", "blue"];
let [firstC, ...restC] = colors;
console.log(firstC);
console.log(...restC);
console.log(restC[0]);//green
console.log(restC[1]);//blue

使用剩余项可以进行数组克隆

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

三 混合解构

let node = {
type: "Identifier",
name: 'foo',
loc: {
    start: {
        line: 1,
        column: 1
    },
    end: {
        line: 1,
        column: 4
    }
},
range: [0, 3]
}

let {
type,
name: localName,
loc: {
    start: {
        line: ll
    },
    end: {
        column: col
    }
},
range: [, second]
} = node;

console.log(type);//Identifier
console.log(localName);//foo
console.log(ll);//1
console.log(col);//4
console.log(second);//3