Array.prototype.map()

let array = [1.9, 2.1, 3.5]

const rounded = array.map((x) => Math.round(x))
console.log(rounded) // [2, 2, 4]

Array.prototype.flat()

let array = [0.1, 2, ["345", [{key: 6},[{key: 7},{key: 8}]], 9]]

console.log(array.flat()) // 攤開1層
// [0.1, 2, Array(3)]
// [0.1, 2, ['345', Array(2), 9]]

console.log(array.flat(2)) // 攤開2層
// [0.1, 2, '345', Array(2), 9]
// [0.1, 2, '345', [{key: 6},[{key: 7},{key: 8}]], 9]

console.log(array.flat(Infinity)) // 全部攤平
// [0.1, 2, '345', {key: 6}, {key: 7},{key: 8}, 9]

Array.prototype.flatMap()

// identical to arr.map(...args).flat(),
// but slightly more efficient than calling those two methods separately.

const arr1 = [1, 2, 1];

const result = arr1.flatMap((num) => (num === 2 ? [2, 2] : 1));

// after map => [1, [2, 2], 1]
// after flat => [1, 2, 2, 1]

console.log(result);
// Expected output: Array [1, 2, 2, 1]

Array.prototype.forEach()

let array = []
array = [0.1, 2, "345", {six: 6}, [{key: 6},{key: 7},{key: 8}], 9]

var types_of_array = []

/* forEach 寫法 */
array.forEach((item) => {
  types_of_array.push(typeof(item))
})

console.log(types_of_array)
// > Array ["number", "number", "string", "object", "object", "number"]

/* reference index 寫法
for (let i = 0; i < array.length; i++) {
	types_of_array.push(typeof(array[i]))
}
*/

Array.from() / Array.fromAsync()

// 可以將 *async* iterable object (e.g., Set, Map) 或是
// array-like (e.g., 有length 且 可用index reference的) object 轉成 array

Array.from("foo");
// [ "f", "o", "o" ]

const set = new Set(["foo", "bar", "baz", "foo"]);
Array.from(set);
// [ "foo", "bar", "baz" ]

 const map = new Map([
  [1, 2],
  [2, 4],
  [4, 8],
]);
Array.from(map);
// [[1, 2], [2, 4], [4, 8]]

const mapper = new Map([
  ["1", "a"],
  ["2", "b"],
]);
Array.from(mapper.values());
// ['a', 'b'];

Array.from(mapper.keys());
// ['1', '2'];

Array.isArray()

// 判定變數是否為array

console.log(Array.isArray([1, 3, 5]));
// Expected output: true

console.log(Array.isArray('[]'));
// Expected output: false

console.log(Array.isArray([]))
// Expected output: true

console.log(Array.isArray(new Array(5)));
// Expected output: true

console.log(Array.isArray(new Int16Array([15, 33])));
// Expected output: false

Array.of()

// 用帶入的arguments建立一個新的array
const array = Array.of("1", 2.3, 4, [5, 6])
console.log(array)
// > Array ["1", 2.3, 4, Array [5, 6]]

console.log(Array.of()) // 空的array
// > Array []

Array.prototype.push() / pop()

const array = Array.of("1", 2.3, 4, [5, 6])

let popped = array.pop()
console.log(popped) // [5, 6]

array.push(Array.of(7, 8, 9))
console.log(array) // ["1", 2.3, 4, [7, 8, 9]]

Array.prototype.reduce() / reduceRight()

https://ithelp.ithome.com.tw/articles/10243053
Map 的目的是遍歷所有 item，經過處理之後，回傳同樣長度的陣列
filter 同樣是遍歷所有 item，但是回傳符合條件的 items。
不過 Reduce 的功能就更為強大了，不僅僅有累加的功能，更可以實現 map 和 filter 的功能。

看了這位大大的文，真的好有幫助。我自己的心得是reduce可以幫助我們在同一次迭代中對array做不同的操作。

// array.reduce(function(total, currentValue, currentIndex, arr), initialValue)

const tv_shows = [
	{name: "AAA", rating: "0+"},
	{name: "BBB", rating: "12+"},
	{name: "CCC", rating: "0+"},
	{name: "DDD", rating: "6+"},
	{name: "EEE", rating: "0+"},
	{name: "FFF", rating: "18+"},
	{name: "GGG", rating: "15+"}
];

const rating_0 = []
const rating_6 = []
const rating_12 = []
const rating_15 = []
const rating_18 = []

tv_shows.reduce((total, currentValue, currentIndex, arr) => {
    if (currentValue.rating === '0+') {
        rating_0.push(currentValue)
    } else if (currentValue.rating === '6+') {
        rating_6.push(currentValue)
    } else if (currentValue.rating === '12+') {
        rating_12.push(currentValue)
    } else if (currentValue.rating === '15+') {
        rating_15.push(currentValue)
    } else if (currentValue.rating === '18+') {
        rating_18.push(currentValue)
    }
})