javascript - 什么是解构赋值及其用途？

Question

我一直在阅读有关 Destructuring assignment 的内容ES6 中引入。

此语法的目的是什么，为什么引入它，以及在实践中如何使用它的一些示例？

Answer 1

What is destructuring assignment ?

解构赋值语法是一种 JavaScript 表达式，可以将数组中的值或对象中的属性解压到不同的变量中。

-MDN

Advantages

A.使代码简洁且更具可读性。

B.我们可以轻松避免重复的破坏表达式。

Some use cases

1。从对象、数组获取变量值

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)


// Nested extraction is possible too:
let obj3 = { foo: { bar: 'bar' } };
let { foo: { bar } } = obj3;
console.log(bar);

2。将一个数组在任意位置与另一个数组组合。

let arr = [2,3,4,5]
let newArr = [0,1,...arr,6,7]
console.log(newArr)

3.仅更改对象中所需的属性

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)

4。创建对象的浅拷贝

let obj = {a:1,b:2,c:3}
let newObj = {...obj}
newObj.a = 'new Obj a'

console.log('Original Object', obj)
console.log('Shallow copied Object', newObj)

5。将参数中的值提取到独立变量中

// Object destructuring:
const fn = ({ prop }) => {
  console.log(prop);
};
fn({ prop: 'foo' });


console.log('------------------');


// Array destructuring:
const fn2 = ([item1, item2]) => {
  console.log(item1);
  console.log(item2);
};
fn2(['bar', 'baz']);


console.log('------------------');


// Assigning default values to destructured properties:

const fn3 = ({ foo="defaultFooVal", bar }) => {
  console.log(foo, bar);
};
fn3({ bar: 'bar' });

6。从对象获取动态键值

let obj = {a:1,b:2,c:3}
let key = 'c'
let {[key]:value} = obj

console.log(value)

7。使用某些默认值从其他对象构建对象

let obj = {a:1,b:2,c:3}
let newObj = (({d=4,...rest} = obj), {d,...rest})
console.log(newObj)

8。交换值

const b = [1, 2, 3, 4];
[b[0], b[2]] = [b[2], b[0]]; // swap index 0 and 2

console.log(b);

9。获取对象的子集

• 9.1 subset of an object :

  const obj = {a:1, b:2, c:3},
  subset = (({a, c}) => ({a, c}))(obj); // credit to Ivan N for this function
  
  console.log(subset);

• 9.2 获得 subset of an object using comma operator and destructuring :

  const object = { a: 5, b: 6, c: 7  };
  const picked = ({a,c}=object, {a,c})
  
  console.log(picked); // { a: 5, c: 7 }

10。进行数组到对象的转换:

const arr = ["2019", "09", "02"],
date = (([year, day, month]) => ({year, month, day}))(arr);

console.log(date);

11。 To set default values in function. (阅读此答案以获取更多信息)

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'})

12。从数组、函数名称、参数数量等获取诸如长度之类的属性。

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);