Copy

1. Two Methods to Copy Arrays

(1) slice

• Can copy the original array

• The newly created array from copying has the same elements as the original array, but references a different address

  let arr = [0, 1, 2, 3];
  let copiedArr = arr.slice();
  console.log(copiedArr); // [0, 1, 2, 3]
  console.log(arr === copiedArr); // false

• Since the addresses are different, even if you add elements to the copied array, they won't be added to the original array

  copiedArr.push(4);
  console.log(copiedArr); // [0, 1, 2, 3, 4]
  console.log(arr); // [0, 1, 2, 3]

(2) Spread Syntax of Array

• A newly added syntax in ES6 that can spread arrays

• Add ... before the variable name assigned to the array

  let arr = [0, 1, 2, 3];
  console.log(...arr); // 0 1 2 3

• First, understand how to create arrays

• If you create two arrays with the same elements as below and assign them to variables respectively, do the two variables reference the same address?

• The answer is, since they are reference data types, they reference different addresses

  let num = [1, 2, 3];
  let int = [1, 2, 3];
  
  console.log(num === int); // false

• Also, if you spread the original array into a new array, it has the same elements as the original array but references different addresses

• As a result, it behaves identically to using the slice() method

  let arr = [0, 1, 2, 3];
  let copiedArr = [...arr];
  console.log(copiedArr); // [0, 1, 2, 3]
  console.log(arr === copiedArr); // false
  
  copiedArr.push(4);
  console.log(copiedArr); // [0, 1, 2, 3, 4]
  console.log(arr); // [0, 1, 2, 3]

2. Two Methods to Copy Objects

(1) Object.assign

let obj = { firstName: 'Ella', lastName: 'Choi' };
let copiedObj = Object.assign({}, obj);

console.log(copiedObj); // { firstName: "Ella", lastName: "Choi" }
console.log(obj === copiedObj); // false

(2) Spread Syntax of Object

• Can be used not only for arrays but also for object copying

  let obj = { firstNmae: 'Ella', lastName: 'Choi' };
  let copiedObj = { ...obj };
  
  console.log(copiedObj); // { firstName: "Ella", lastName: "**Choi**" }
  console.log(obj === copiedObj); // false