传播和休息运算符

const nums = [5,6,7];
const newnums = [1,2, nums[0],nums[1],nums[2]];
console.log(newnums); // [ 1, 2, 5, 6, 7 ]

is reduced to 

const nums = [5,6,7];
const newnums = [1,2, ...nums];
console.log(newnums); // [ 1, 2, 5, 6, 7 ]
console.log(...newnums); // 1 2 5 6 7

const arr1 = [1,2,3,4,5];
const arr2 = [6,7,8,9];

let nums = [...arr1,...arr2];
nums; // [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]


const firstname = "peter";
const fullname = [...firstname,' ',"p."];
fullname; // [ 'p', 'e', 't', 'e', 'r', ' ', 'p.' ]

console.log(...firstname); // 'p' 'e' 't' 'e' 'r'

const girl = {
  name: 'melania',
  friends: ['alina', 'alice', 'ayesha', 'anamika', 'anaya']
};

const frnz = [...girl.friends];
console.log(frnz); // [ 'alina', 'alice', 'ayesha', 'anamika', 'anaya' ]
console.log(girl.friends); // [ 'alina', 'alice', 'ayesha', 'anamika', 'anaya' ]

let male = {
  "firstname": "gangadhar",
  "lastname": "shaktimaan"
}

let female = {
  "firstname": "geeta",
  "lastname": "vishwas"
}

let x = {...male, born: 1950};
let y = {...female, born: 1940};

x; // { firstname: 'gangadhar',   lastname: 'shaktimaan',   born: 1950 }
y; // { firstname: 'geeta',  lastname: 'vishwas',  born: 1940 }```




## shallow copy of objects:

let male = {
  "firstname": "gangadhar",
  "lastname": "shaktimaan"
}

let character = {...male, firstname: 'wonderwoman'};

male;         // { firstname: 'gangadhar', lastname: 'shaktimaan' }
character;    // { firstname: 'wonderwoman', lastname: 'shaktimaan' }

- first name for character object is changed although it was extracted from male object

## rest syntax collects all the elements after the last elements into an array. doesn't include any skipped elements. 
- hence, it should be the last element in the destructuring assignment.
- there can be only one rest in any destructuring assignment.

- rest also works with objects. only difference is that elements will be collected into a new object instead of an arrray.

- although both look same, but they work differently based on where they are used.