const nestedArr = [1, 2, [3, 4, [5, 6, [7, [8], 9]]], 10]
console.log(nestedArr.flat())
// [1, 2, 3, 4, [5, 6, [7, [8], 9]], 10]
console.log(nestedArr.flat(2))
// [1, 2, 3, 4, 5, 6, [7, [8], 9], 10]
console.log(nestedArr.flat(3))
// [1, 2, 3, 4, 5, 6, 7, [8], 9, 10]
console.log(nestedArr.flat(4))
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
console.log(nestedArr.flat(Infinity))
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

const myArr = [1, 2, 3]
myArr
 .map(n => [n * n]) // [[1], [4], [9]]
 .flat() // [1, 4, 9]

// 用flatMap可以一步到位
myArr.flatMap(n => [n * n]) // [1, 4, 9]

// 假如我們想要?jiǎng)h除掉原數(shù)組里面所有的負(fù)數(shù)，同時(shí)將單數(shù)轉(zhuǎn)換為一個(gè)復(fù)數(shù)和1
const a = [5, 4, -3, 20, 17, -33, -4, 18]
//    |\ \ x  |  | \  x  x  |
//    [4,1, 4,  20, 16,1,     18]
a.flatMap(n =>
 (n < 0) ? []: // 刪除負(fù)數(shù)
 (n % 2 == 0) ? [n] : // 保留復(fù)數(shù)
         [n - 1, 1] // 單數(shù)變?yōu)橐粋€(gè)復(fù)數(shù)和1
)
// [4, 1, 4, 20, 20, 16, 1, 18]

// key-value pairs數(shù)組
const entriesArr = [['k1', 1], ['k2', 2]]
console.log(Object.fromEntries(entriesArr)
// {k1: 1, k2: 2}

const entriesMap = new Map([
 ['k1', 1],
 ['k2', 2]
]) // {"k1" => 1, "k2" => 2}
console.log(Object.fromEntries(entriesMap))
// {k1: 1, k2: 2}

const iteratorObj = {
 [Symbol.iterator]: function () {
  const entries = [['k1', 1], ['k2', 2]]
  let cursor = 0

  return {
   next() {
    const done = entries.length === cursor
    
    return {
     value: done ? undefined : entries[cursor++],
     done
    }
   }
  }
 }
}
Object.fromEntries(iteratorObj) // {k1: 1, k2: 2}

const studentMap = {
 student1: {grade: 80},
 student2: {grade: 50},
 student3: {grade: 100}
}
const goodStudentMap = Object.fromEntries(
 Object
  .entries(studentMap)
  .filter(([_, meta]) => meta.grade >= 60)
)
console.log(goodStudentMap)
// {student1: {grade: 80}, student3: {grade: 100}}

const greeting = '  Hello world! '
console.log(greeting.trimStart())
// 'Hello world! '

const greeting = ' Hello world! '
console.log(greeting.trimEnd())
// ' Hello world!'

console.log(Symbol('desc').description)
// desc
console.log(Symbol.for('desc').description)
// desc

// 一些內(nèi)置的Symbol也有這個(gè)屬性
console.log(Symbol.iterator.description)
// Symbol.iterator

// 如果初始化時(shí)沒(méi)有帶description，這個(gè)屬性會(huì)返回一個(gè)undefined，因?yàn)檫@樣才說(shuō)這個(gè)屬性是可選的
console.log(Symbol().description)
// undefined

// 這個(gè)屬性是只讀的，不能被設(shè)置
Symbol.iterator.description = 'mess it'
console.log(Symbol.iterator.description)
// Symbol.iterator

var s1 = Symbol("desc")
var s2 = Symbol("desc")
console.log(s1.description === s2.description)
// true
console.log(s1 === s2)
// false

// ECMAScript2019之前，你一定要在catch里面聲明error，否則會(huì)報(bào)錯(cuò)
try {
 ...
} catch (error) {

}
// 可是有時(shí)候，你確實(shí)用不到這個(gè)error對(duì)象，于是你會(huì)寫(xiě)這樣的代碼
try {
 ...
} catch (_) {
 ...
}

// ECMAScript2019后，你可以直接這樣寫(xiě)了
try {
 ...
} catch {
 ...
}

let testJSONObj
try {
 testJSONObj = JSON.prase(testStr)
} catch {
 testJSONObj = {}
}
console.log(testJSONObj)

var users = [
 {name: 'Sean', rating: 14},
 {name: 'Ken', rating: 14},
 {name: 'Jeremy', rating: 13}
]
users.sort((a, b) => a.rating - b.rating)
// 非穩(wěn)定的排序結(jié)果可能是
// [
//  {name: 'Jeremy', rating: 13}, 
//  {name: 'Ken', rating: 14}, 
//  {name: 'Sean', rating: 14}
// ]
// 雖然Sean和Ken具有同樣的rating，可是非穩(wěn)定的排序不能保證他們兩個(gè)的順序在排序后保持不變

// ECMAScript2019后，Array.sort將是一個(gè)穩(wěn)定的排序，也就是說(shuō)它可以保證Sean和Ken兩個(gè)人的順序在排序后不變
// [
//  {name: 'Jeremy', rating: 13}, 
//  {name: 'Sean', rating: 14}, 
//  {name: 'Ken', rating: 14}
// ]

function hello() {
 console.log('hello word')
}

console.log(hello.toString())
//'function hello() {\nconsole.log('hello word')\n}'

console.log(hello.toString())
//"function hello() {
// console.log('hello word')
//}"