什么是解构赋值?
解构赋值语法是一种JavaScript表达式,它可以将数组中的值或对象中的属性解包到不同的变量中。
- 来自MDN
优点
一些使用案例
从对象和数组中获取变量的值
```javascript
let obj = { 'a': 1,'b': {'b1': '1.1'}}
let {a,b,b:{b1}} = obj
console.log('a--> ' + a, '\nb--> ', b, '\nb1---> ', b1)
let obj2 = { foo: 'foo' };
let { foo: newVarName } = obj2;
console.log(newVarName);
let arr = [1, 2, 3, 4, 5]
let [first, second, ...rest] = arr
console.log(first, '\n', second, '\n', rest)
// 嵌套提取也是可能的:
let obj3 = { foo: { bar: 'bar' } };
let { foo: { bar } } = obj3;
console.log(bar);
```
只更改对象中的所需属性
```javascript
let arr = [{a:1, b:2, c:3},{a:4, b:5, c:6},{a:7, b:8, c:9}]
let op = arr.map( ( {a,...rest}, index) => ({...rest,a:index+10}))
console.log(op)
```
将参数中的值提取到独立变量中
```javascript
// 对象解构:
const fn = ({ prop }) => {
console.log(prop);
};
fn({ prop: 'foo' });
console.log('------------------');
// 数组解构:
const fn2 = ([item1, item2]) => {
console.log(item1);
console.log(item2);
};
fn2(['bar', 'baz']);
console.log('------------------');
// 为解构的属性分配默认值:
const fn3 = ({ foo="defaultFooVal", bar }) => {
console.log(foo, bar);
};
fn3({ bar: 'bar' });
```
从对象中获取动态键值
```javascript
let obj = {a:1,b:2,c:3}
let key = 'c'
let {[key]:value} = obj
console.log(value)
```
交换值
```javascript
const b = [1, 2, 3, 4];
[b[0], b[2]] = [b[2], b[0]]; // 交换索引0和2的值
console.log(b);
```
获取对象的子集
```javascript
const obj = {a:1, b:2, c:3},
subset = (({a, c}) => ({a, c}))(obj); // 感谢Ivan N提供的函数
console.log(subset);
```
使用逗号运算符和解构来获取对象的子集
```javascript
const object = { a: 5, b: 6, c: 7 };
const picked = ({a,c}=object, {a,c})
console.log(picked); // { a: 5, c: 7 }
```
数组转对象
```javascript
const arr = ["2019", "09", "02"],
date = (([year, day, month]) => ({year, month, day}))(arr);
console.log(date);
```
在函数中设置默认值
```javascript
function someName(element, input, settings={i:"#1d252c", i2:"#fff", ...input}){
console.log(settings.i);
console.log(settings.i2);
}
someName('hello', {i: '#123'});
someName('hello', {i2: '#123'});
```
从数组中获取属性,如`length`,函数名,参数数量等
```javascript
let arr = [1,2,3,4,5];
let {length} = arr;
console.log(length);
let func = function dummyFunc(a,b,c) {
return 'A B and C';
}
let {name, length:funcLen} = func;
console.log(name, funcLen);
```