javascript-7: Array

Array

정의

• 순서가 있는 리스트

1. Declaration

const arr1 = new Array();
const arr2 = [1,2];

2. Index position

const fruits=['🍎','🍌'];

console.log(fruits);

console.log(fruits.length);

console.log(fruits[0]);

console.log(fruits[1]);

console.log(fruits[2]);      //배열에 없는 인덱스를 호출하면 undefined

console.log(fruits[fruits.length-1]);    //뒤에서 첫번째

3. Looping over an array

3-1 for

const fruits=['🍎','🍌'];

for(leti=0; i < fruits.length; i++){
  console.log(fruits[i]);
}

3-2 for of

const fruits=['🍎','🍌'];

for(fruit of fruits){
  console.log(fruit);
}

3-3 forEach

const fruits=['🍎','🍌'];

fruits.forEach((fruit) => console.log(fruit));

**4. Addtion, deletion, copy

4-1 push: add an item to the end

const fruits=['🍎','🍌'];

fruits.push('🍓','🍑');

console.log(fruits);

4-2 pop: remove an item from the end

fruits.pop();
console.log(fruits);

4-3 unshift: add an item to the benigging

fruits.unshift('🍓','🍋');

console.log(fruits);

4-4 shift: remove an item from the benigging

fruits.shift();
console.log(fruits);

* shift, unshift는 push와pop보다 느림.

4-5 splice: remove an item by index position

const fruits=['🍎','🍌'];
fruits.push('🍓','🍑','🍋');
console.log(fruits);

fruits.splice(1,1);     //1이라는 index부터 1개만 제거
console.log(fruits);

fruits.splice(1,0,'🍏','🍉');  //1이라는 index에 0개를 제거하고, 사과와 수박을 추가함
console.log(fruits);

4-6 concat = combine two arrays

const fruits2 = ['🍐','🥥'];
const newFruits = fruits.concat(fruits2);

console.log(newFruits);

5. Searching

5-1 indexOf : find the index (몇번째인지)

console.log(fruits);

console.log(fruits.indexOf('🍎'));

console.log(fruits.indexOf('🍉'));

console.log(fruits.indexOf('🥥')); //배열에 없는것을 선택했을 때

5-2 includes(들어있는지 없는지)

console.log(fruits.includes('🍉'));
 
console.log(fruits.includes('🥥'));  //배열에 없는것을 선택했을 때

5-3 last IndexOf

fruits.push('🍎');

console.log(fruits);

console.log(fruits.indexOf('🍎'));   //indexof는 가장 처음에 있는것(0) 하나만 나타냄
console.log(fruits.lastIndexOf('🍎')); //lastindexof는 가장 마지막에있는것 하나만 나타냄
console.log(fruits.lastIndexOf('🥥')); //배열에 없는것을 선택했을 때

5-4 arr.find(fn)/ arr.findindex(fn)

• 보다 복잡한 연산이 될수있도록 함수를 인수로 받음
• 첫번째 true 값만 반환하고 끝
• 만약 없으면 undefined를 반환
• index는 index값을 반환하고, 값이 없으면 -1를 반환함

let arr = [1,2,3,4,5];
const result = arr.find((item) => {
  return item % 2 === 0;
});
console.log(result);