解构使得将数组中的值或对象中的属性解包为不同的变量成为可能。

优点

• 使代码简洁且更具可读性。
• 我们可以轻松避免重复的解构表达式。

一些用例

1. 从对象、数组中获取变量值。

let array = [1, 2, 3, 4, 5];
let [first, second, ...rest] = array;
console.log(first, second, rest);
//expected output: 1 2 [3,4,5]

let objectfoo = { foo: 'foo' };
let { foo: newvarname } = objectfoo;
console.log(newvarname);
//expected output: foo

// nested extraction
let objectfoobar = { foo: { bar: 'bar' } };
let { foo: { bar } } = objectfoobar;
console.log(bar);
//expected output: bar

1. 仅更改对象中所需的属性。

  let array = [
    { a: 1, b: 2, c: 3 },
    { a: 4, b: 5, c: 6 },
    { a: 7, b: 8, c: 9 },
  ];
  let newarray = array.map(({ a, ...rest }, index) => ({
    a: index + 10,
    ...rest,
  }));
  console.log(newarray);
/* expected output: [
  {
    "a": 10,
    "b": 2,
    "c": 3    
  },
  {
    "a": 11,
    "b": 5,
    "c": 6
  },
  {
    "a": 12,
    "b": 8,
    "c": 9
  }
]*/

1. 将参数中的值提取到独立变量中。

// object destructuring:
  const objectdestructure = ({ property }: { property: string }) => {
    console.log(property);
  };
  objectdestructure({ property: 'foo' });
  //expected output: foo

  // array destructuring:
  const arraydestructure = ([item1, item2]: [string, string]) => {
    console.log(item1 , item2);
  };
  arraydestructure(['bar', 'baz']);
  //expected output: bar baz


// assigning default values to destructured properties:
  const defaultvaldestructure = ({
    foo = 'defaultfooval',
    bar,
  }: {
    foo?: string;
    bar: string;
  }) => {
    console.log(foo, bar);
  };
  defaultvaldestructure({ bar: 'bar' });
//expected output: defaultfooval bar

1. 从对象中获取动态键值。

const obj = { a: 1, b: 2, c: 3 };
const key = 'c';
let { [key]: val } = obj;
console.log(val);
//expected output: 3

1. 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);
//expected output : [3,2,1,4]

1. 获取对象的子集。

const obj = { a: 5, b: 6, c: 7 };
const subset = (({ a, c }) => ({ a, c }))(obj);
console.log(subset); 
// expected output : { a: 5, c: 7 }

1. 进行数组到对象的转换。

const arr = ["2024", "17", "07"],
      date = (([year, day, month]) => ({year, month, day}))(arr);
console.log(date);
/* expected output: {
  "year": "2024",
  "month": "07",
  "day": "17"
} */

1. 在函数中设置默认值。

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'});
/* expected output: 
#123
#fff
#1d252c
#123 
*/

1. 获取数组长度、函数名称、参数数量等属性。

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);
/* expected output : 
5
dummyfunc 3
*/

1. 组合数组或对象。

//combining two arrays
const alphabets = ['A','B','C','D','E','F'];
const numbers = ['1','2','3','4','5','6'];
const newArray = [...alphabets, ...numbers]
console.log(newArray);
//expected output: ['A','B','C','D','E','F','1','2','3','4','5','6']

//combining two objects
const personObj = { firstname: "John", lastname: "Doe"};
const addressObj = { city: "some city", state: "some state" };
const combinedObj = {...personObj, ...addressObj};
console.log(combinedObj);
/* expected output: {
    "firstname": "John",
    "lastname": "Doe",
    "city": "some city",
    "state": "some state"
} */

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