クコと何夜も格闘した結果、JavaScript と ES に関する重要な知識を 25 個まとめました。

function funValue(value) {
    value="dada";
    alert(value); // dada
    alert(argument[0]); // 非严格模式：dada
    // 严格模式模式 dadaqianduan}

funValue('dadaqianduan');复制代码

// es5let dada = function(type) {    this.type = type
}

dada.prototype.study = function() {    console.log('魔王哪吒');
}let da1 = new dada('程序员')let da2 = new dada('It')

da1.constructor.prototype.study = function() {    console.log('dadaqianduan');
}
da1.study()复制代码

// es6class Da {  constructor(name) { // 构造函数内写属性
    this.name = name;
  }
  eat() { // 构造函数外写方法
    console.log('i eat')
  }
}const da1 = new Da('da1');console.log(da1.name); // da1console.log(da1);复制代码

var daObj = {
 get val() {
  return ;
 },
 set val(value) {
 }
}复制代码

var da = {    a: 1,
    get val(){        return this.a + 1;
    }
}console.log(da.val);//2da.val = 100;console.log(da.val);//2class Da { constructor(type) {  this.type = type
 }
 get age() {  return 1
 }
 set age(val) {  this.realAge = val
 }
 eat() {  console.log('i am eat')
 }
}let da1 = new Da('da1')console.log(da1.age)
da1.age = 1console.log(da1.realAge)复制代码

class Da {
 constructor(type, age) {
  this.type = type
  this.age1 = age
 }
 get age() {
  return this._age
 }
 set age(val) {
  this._age = val
 }
}复制代码

class myHTMLElement {
 constructor(element) {
  this.element = element
 }
 get html() {
  return this.element.innerHTML
 }
 set html(value) {
  this.element.innerHTML = value
 }
}复制代码

let myName = 'dada'
class Da {
 constructor(type) {
  this.type = type
 }
 get name() {
  return myName
 }
 set name(val) {
  myName = val
 }
}复制代码

let Da = function (type) { this.type = type this.eat = function() {  console.log('i eat')
 }
}
Da.study = function(book) { console.log('i book');
}复制代码

let Da = function(type) { this.type = type
}
Da.prototype.eat = function() {
 Da.walk() console.log('i am')
}
Da.walk = function(){ console.log('walk')
}let da1 = new Da('da1')
da1.eat()// walk// i am复制代码

class Da {
 constructor(type) {
  this.type = type
 }
 eat() {
  console.log('i eat')
 }
 static study() {
  console.log('i study')
 }
}复制代码

// 定义一个父类
let Da = function(type) {
 this.type = type
}
// 定义方法
Da.prototype.eat = function() {
 console.log('i am')
}
// 定义静态方法
Da.study = function(book) {
 console.log('i study')
}
// 定义子类
let Da1 = function() {
 // 初始化父类
 Da.call(this, 'da1');
 this.run = function() {
  console.log('i run')
 }
}
// 继承
Da1.prototype = Da.prototype复制代码

class Da { constructor(type) {  this.type = type
 }
 eat() {  // Da.walk();
  console.log('i eat')
 } static walk(){  console.log('i walk')
 }
}class da extends Da { // 构造函数
 //constructor (type) {
  //super(type)
 //}
 run() {  console.log('i run')
 }
}let da1 = new da('da1')复制代码

// 类的声明
let Da = function(type) {
 this.type = type
 this.eat = function() {
  console.log('i eat');
 }
}

let da = new Da('da');复制代码

// prototype
let Da = function(type) {
 this.type = type
}
Da.prototype.eat = function() {
 console.log('i eat')
}
let da1 = new Da('da1')复制代码

class Da {
 // 构造函数
 constructor(type) {
  this.type = type
 }
 // 方法
 walk() {
  console.log('i walk')
 }
}
let da = new Da('da');
// console.log(typeof Da); function复制代码

// 参数默认值
function da (x,y,z) {
}
function sum() {
 let num = 0
 Array.prototype.forEach.call(arguments, function(item){
  num += item * 1
 })
 Array.from(arguments).forEach(function(item){
  num += item * 1
 })
 return num
}复制代码

// 不确定
function sum(...nums) {
 let num = 0
 nums.forEach(function(item){
  num += item * 1
 })
 return num
}
console.log(sum(1,2,3,4,5))复制代码

function sum () {
  let num = 0
  Array.prototype.forEach.call(arguments, function (item) {
    num += item * 1
  })
  return num
}

function sum (...nums) {
  let num = 0
  nums.forEach(function (item) {
    num += item * 1
  })
  return num
}复制代码

() => {}
// function Da() {}
// let da = function() {}
let da = () => {
 console.log('hello')
}
da()

let da = name => {}复制代码

const materials = [  'Hydrogen',  'Helium',  'Lithium',  'Beryllium'];console.log(materials.map(material => material.length));// expected output: Array [8, 6, 7, 9]复制代码

console.log(sum(...[4]))
console.log(sum.apply(null, [4]))复制代码

function myFunc(a, b, ...args) {
 console.log(a); // 22
 console.log(b); // 98
 console.log(args); // [43, 3, 26]
};
myFunc(22, 98, 43, 3, 26);复制代码

function myFunc(x, y, ...params) { // used rest operator here
  console.log(x);  console.log(y);  console.log(params);
}var inputs = ["a", "b", "c", "d", "e", "f"];
myFunc(...inputs); // used spread operator here// "a"// "b"// ["c", "d", "e", "f"]复制代码

var obj1 = { foo: 'bar', x: 42 };var obj2 = { foo: 'baz', y: 13 };var clonedObj = { ...obj1 };// Object { foo: "bar", x: 42 }var mergedObj = { ...obj1, ...obj2 };// Object { foo: "baz", x: 42, y: 13 }复制代码

let x = 'da1';
let y = 'da2';
let obj = {
 x,
 y
}
console.log(obj);

// 结果
{x:'da1',y:'da2'}复制代码

let x=1; let y=2; let z=3
let obj = {
 'x': x,
 y,
 [z+y]: 4,
 * hello() { // 异步
  console.log('dada')
 }
}
// function* functionName() {}
obj.hello()复制代码

const s = new Set();

[2, 3, 5].forEach(x => s.add(x));

Set 函数可以接受一个数组（或类似数组的对象）作为参数，用来初始化

const set = new Set([1, 2, 3, 4, 4]);复制代码

var arr = [1,1,2,2,3,3]; // step1:数组转集合
var s = new Set(arr); // 已经去掉重复值，当前不是数组，而集合
s.size; // 3
// step2:集合转数组
console.log([...s]); // 1,2,3;

// Array.form 方法可以将 Set 结构转为数组
const items = new Set([1, 2, 3]);
const arr = Array.from(items);

function dada(array) {
  return Array.from(new Set(array));
}

dada([1, 1, 2])复制代码

let set = new Set([1, 2, 3, 4, 4]);

// 添加数据 
let addSet = set.add(5);
console.log(addSet); // Set(5) {1, 2, 3, 4, 5}

// 删除数据 
let delSet = set.delete(4);
console.log(delSet); // true

// 查看是否存在数据 4
let hasSet = set.has(4);
console.log(hasSet); // false

// 清除所有数据
set.clear();
console.log(set); // Set(0) {}复制代码

let a = new Set([1, 2, 3]);
let b = new Set([4, 3, 2, 1]);

// 并集
let union = new Set([...a, ...b]);
// Set {1, 2, 3, 4}

// 交集
let intersect = new Set([...a].filter(x => b.has(x)));
// set {1, 2, 3}

// 差集
let difference = new Set([...b].filter(x => !a.has(x)));
// Set {4}复制代码

let map = new Map()
map.set(1, 2)
map.set(3, 4)
map.set(1, 3)
console.log(map)

创建
var da = new Map();
var jeskson = {};
遍历
da.forEach(function(value,key,map){}
长度
da.size
删除
//da.delete() 删除key，全部清楚da.clear()
新增
da.set(key,value)
da.has(查索引值)

da.forEach((value,key) =>{
})

for( let [key, value] of map){}

// let map = new Map( [[1,2], [3,4]] )

map的key任意都可以
let o = function() {
 console.log('o')
}
map.set(o, 3)
console.log(map.get(o)); // 3复制代码

// map.js
var Dictionary = function() {
 var items = {};
 // 检查键
 this.has = function(key) {
  return key in items;
 }
 // 添加键值对
 this.set = function(key, value){
  items[key] = value;
 }
 // 通过键移除元素
 this.delete = function(key) {
  if(this.has(key)){
   delete items[key]
   return true
  }
  return false
 }
 // 键获取值
 this.get = function(key){
  return this.has(key) ? items[key] : undefined;
 }
 // 列表返回字典值
 this.values = function() {
  var values = [];
  for(var k in items) {
   if(this.has(k)) {
    values.push(items[k])
   }
  }
  return values;
 }
 // 获取全部键名
 this.keys = function() {
  return Object.keys(items);
 }
 // 获取私有变量items
 this.getItems = function() {
  return items;
 }
}复制代码

const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };

const returnedTarget = Object.assign(target, source);

console.log(target);
// expected output: Object { a: 1, b: 4, c: 5 }

console.log(returnedTarget);
// expected output: Object { a: 1, b: 4, c: 5 }

> Object { a: 1, b: 4, c: 5 }
> Object { a: 1, b: 4, c: 5 }复制代码

Object.assign(target, ...sources)复制代码

target复制代码

sources复制代码

const obj = { a: 1 };
const copy = Object.assign({}, obj);
console.log(copy); // { a: 1 }复制代码

Object.assign(undefined) // 报错
Object.assign(null) // 报错

let obj = {a: 1};
Object.assign(obj, undefined) === obj // true
Object.assign(obj, null) === obj // true

const obj1 = {a: {b: 1}};
const obj2 = Object.assign({}, obj1);

obj1.a.b = 2;
obj2.a.b // 2

const target = { a: { b: 'c', d: 'e' } }
const source = { a: { b: 'hello' } }
Object.assign(target, source)
// { a: { b: 'hello' } }

const source = {
  get foo() { return 1 }
};
const target = {};

Object.assign(target, source)
// { foo: 1 }复制代码

function ObjectAssign(target, ...sources) {
 // 对第一个参数的判断，不能为undefined和null
 if(target === undefined || target === null) {
  throw my TypeError('error');
 }
 // 将第一个参数转为对象(不是对象转换为对象)
 const targetObj = Object(target);
 // 将源对象自身的所有可枚举属性复制到目标对象
 for(let i = 0; i<sources.length; i++){
  let source = sources[i];
  // 对于undefined和null在源中不会报错，会直接跳过
  if(source !== undefined && source !== null) {
   // 将源象转换成对象
   // 需要将源自身的可枚举数据进行复制
   // Reflect.ownKeys(obj)
   const keysArray = Reflect.ownKeys(Object(source));
   for (let nextIndex = 0; nextIndex < keysArray.length; nextIndex++) {
    const nextKey = keysArray[nextIndex];
    // 去除不可枚举属性
    const desc = Object.getOwnPropertyDescriptor(source,nextKey);
    if(desc!==undefined&&desc.enumerable){
     targetObj[nextKey] = source[nextKey];
    }
   }
  }
 }
 return targetObj;
}
if(typeof Object.myAssign !== &#39;function&#39;){
 Object.defineProperty(Object, &#39;myAssign&#39;, {
  value: ObjectAssign,
  writable: true,
  enumerable: false,
  configurable: true
 });
}复制代码

let da1 = { name: 'da1', age: 1}let da2 = { name: 'da2', study: {  title: 'web'
 }
}let da3 = Object.assign(da1,da2);console.log(da3);// {// name: 'da2',// age: 1,// study: { title: 'web' }// }console.log( da1 === da3); // trueda2.name = 'da22';
da2.study.title = 'web2';console.log(da2);// {// name: 'da22',// study: { title: 'web2' }// }console.log(da1);// {// age: 1,// name: 'da2',// study: { title: 'web2' }// }复制代码

let da1 = { name: 'da1', age: 1}let da2 = { a: Symbol('dadaqianduan'), b: null, c: undefined}let da3 = Object.assign(da1, da2);console.log(da3);// {// name: 'da1',// age: 1,// a: Symbol('dadaqianduan'),// b: null,// c: undefined// }console.log(da1 === da3); // true复制代码

let map = new Map([iterable])
// Map是用来实现字典的功能-Object键值对复制代码

// ES5 code
var
  key1 = 'one',
  obj = {
    two: 2,
    three: 3
  };

obj[key1] = 1;

// obj.one = 1, obj.two = 2, obj.three = 3

// ES6 code
const
  key1 = 'one',
  obj = {
    [key1]: 1,
    two: 2,
    three: 3
  };

// obj.one = 1, obj.two = 2, obj.three = 3

// ES6 code
const
  i = 1,
  obj = {
    ['i' + i]: i
  };

console.log(obj.i1); // 1复制代码

const module_string = `dadaqianduan`复制代码

// 要创建跨越两行的字符串，必须\在行尾使用字符const dada = 
 'dada \
  dadaqianduan'
  // 呈现效果：在两行上创建一个字符串，但是仅在一行上呈现复制代码

const string = 
 'dada 魔王哪吒\n \
  dadaqianduan'复制代码

const dada = `dadaqianduan 
魔王哪吒`复制代码

const da = `First
            Second`复制代码

const da = `dadaqianduan ${mydada}`

${}里面可以添加任何东西

const da1 = `dada ${1+2+3}`
const da2 = `dada ${dafun() ? 'x' : 'y'}`复制代码

let da = ['hello', 'world']
let [firstName, surName] = da
cosole.log(firstName, surName);复制代码

let [da1, da2] = [];console.log(da1); // undefinedconsole.log(da2); // undefined复制代码

let [da1= 'da1', da2 = 'da2']=['dadaqianduan]

console.log(da1); // dadaqianduan
console.log(da2); // da2复制代码

var myArray = ['a', 'b', 'c'];var
  one   = myArray[0],
  two   = myArray[1],
  three = myArray[2];// one = 'a', two = 'b', three = 'c'复制代码

const [one, , three] = myArray;// one = 'a', three = 'c'复制代码

const [one, ...two] = myArray;// one = 'a', two = ['b, 'c']复制代码

const myObject = {  one:   'a',  two:   'b',  three: 'c'};// ES6 destructuring exampleconst {one: first, two: second, three: third} = myObject;// first = 'a', second = 'b', third = 'c'复制代码

var a = 1, b = 2;// ES5 swapvar temp = a;
a = b;
b = temp;// a = 2, b = 1// ES6 swap back[a, b] = [b, a];// a = 1, b = 2[b, c, d, e, a] = [a, b, c, d, e];复制代码

function dada(param = {}) {复制代码

function f() {  return [1, 2, 3];
}const [a, b, c] = f();// a = 1, b = 2, c = 3复制代码

var {da} = {bar: 'dada'}console.log(da)// undefined复制代码

var {da:{bar}} = {dada: 'dadaqianduan'}// Exception复制代码

var key = 'dadaqianduan'var { [key]: foo } = { dadaqianduan: 'bar' }console.log(foo)// 'bar'复制代码

var {da=3} = { da: 2 }console.log(da)// 2var {da=3} = { da: undefined }console.log(da)// 3var {da=3} = { bar: 2 }console.log(da)// 3var [a] = []console.log(a)//  undefinedvar [b=10] = [undefined]console.log(b)//  10var [c=10] = []console.log(c)//  10function da () {  return {    x: 1,    y: 2
  }
}var {x, y} = da()console.log(x)// 1console.log(y)// 2复制代码

function loadScript(src) { return new Promise((resolve, reject) => {  let script = document.createElement('script')
  script.src = src
  script.onload = () => resolve(src)
  script.onerror = (err) => reject(err)  document.head.append(script)
 })
}复制代码

function loadScript(src) { let script = document.createElement('script');
 script.src = src; document.head.append(script)
}复制代码

var promise = new Promise(function(resolve, reject){
 resolve('传递给then的值')
})
promise.then(function(value){
 console.log(value)
},function(error){
 console.error(error)
})复制代码

const promise = new Promise((resolve, reject) => {
 setTimeout(() => {
  resolve('da');
 }, 200);
});

promise.then((value) => {
 console.log(value);
});

console.log(promise);复制代码

new Promise(function (resolve,reject){...});复制代码

function myAsyncFunction(url) { return new Promise((resolve, reject) => {  const xhr = new XMLHttpRequest();
  xhr.open('GET',url);
  xhr.onload = () => resolve(xhr.responseText);
  xhr.onerror = () => reject(xhr.statusText);
  xhr.send();
 });
}复制代码

var target = {}var handler = {}var proxy = new Proxy(target, handler)
proxy.a = 'b'console.log(target.a)// 'b'console.log(proxy.c === undefined)// true复制代码

var person = {  name: "dada",  age: 17};

person = new Proxy(person, {
  set(target, property, value) {    if (value === 18) {      console.log("Congratulations! You are of legal age");      Reflect.set(target, property, value);      return true;
    }
  }
});

person.age = 18;if (value < 13 && value > 99) {  throw new Error(&#39;The age should be between 13 and 99&#39;)
} else {  Reflect.set(target, property, value)
}复制代码

let p = new Proxy(target, handler)复制代码

console.log(o.dada || '')复制代码

let o = {
 name： 'dada', age: 1}let handler = {
 get(obj, key) {  return Reflect.has(obj, key)?obj[key]:''
 }
}let p = new Proxy(o, handler)console.log(p.from)复制代码

for (let [key] of Object.entries(response.data)) { 
 Object.defineProperty(response.data, key, {
  writable: false
 })
}复制代码

let data = new Proxy(response.data, {
 set(obj, key, value) {   return false
 }
})复制代码

// Validator.jsexport default(obj, key, vlaue) => { if(Reflect.has(key) && value > 20) {
  obj[key] = value
 }
}import Validator from './Validator'let data = new Proxy(response.data, { set: Validator
})复制代码

let validator = {
 set(target, key, value) {  if(key === 'age') {   if(typeof value !== 'number' || Number.isNaN(value)) {    throw new TypeError('Age must be a number')
   }   if(value<=0){    throw new TypeError(&#39;Age must be a positive number&#39;)
   }
  }  return true
 }
}const person = { age: 12 }const proxy = new Proxy(person,validator)
proxy.age = &#39;dada&#39; // TypeError numberproxy.age = NaNproxy.age = 0 // positive numberproxy.age = 3复制代码

class Component { constructor() {  this.proxy = new Proxy({   id: Math.random().toString(36).slice(-8)
  })
 }
 get id() {  return this.proxy.id
 }
}复制代码

function * dada() {
 for(let i=0; i<2; i++ {
  yield console.log(i);
 }
}

const da = dada()
da.next()
da.next()复制代码

function* dada() {
console.log('dadaqianduan');
}
dada(); // 没有执行函数
 
如需要输出，改为：
var da = dada();
da.next();复制代码

var count_id = 0;
function dada_id() {
count_id ++;
return count_id;
}复制代码

Generator.prototype.next()
返回一个由 yield表达式生成的值。

Generator.prototype.return()
返回给定的值并结束生成器。

Generator.prototype.throw()
向生成器抛出一个错误。复制代码

function *da() {
}

function* da(){
}复制代码

function * da() {
 yield 1
 yield 2
 yield 3
}
var d = da()
console.log(d.next()) // {value:1,done:false}
console.log(d.return()) // {value:undefined,done:true}
console.log(d.next()) // {value:undefined,done:true}复制代码

function * da() {
 yield 1
 yield 2
 yield 3
}
var d = da()
console.log(d.nex()) // {value:1,done:false}
console.log(d.return(100)) // {value:100,done:true}
console.log(d.next()) // {value:undefined,done:true}复制代码

function* genFunc(){...}
const genObj = genFunc();复制代码

const genFunc = function* () {...}
const genObj = genFunc();复制代码

const obj = {
 * generatorMethod(){
  ...
 }
}
const genObj = obj.generatorMethod();复制代码

class MyClass{
 * generatorMethod(){
  ...
 }
}
const myInst = new MyClass();
const genObj = myInst.generatorMethod();复制代码

authors[Symbol.iterator] = function(){
 // this
 return {
  next(){
   return{
    done:false,
    value:1
   }
  }
 }
}复制代码

导出变量或者常量

export const da = 'dadaqianduan'
export let da1 = 'da1'
export var da2 = 'da1'

const name = 'dada'
let name1 = 'dada1'
export {
 name,
 name1
}

导出函数
export function da(value){
 console.log(value)
}

const da = (value) => {
 console.log(value);
}

export {
 da
}

导出Object
export({
 name: 'da1',
 message: 'dadaqianduan'
})

let da = {
 name: 'name1'
}
export {
 da
}

导出Class
class Da {
 constructor(){
  this.id = 1
 }
}
export {
 Da
}

export class Da {
 constructor() {
  this.id = 1
 }
}

修改导出名称
const name = 'da1'
export {
 name as cname
}
export default name复制代码

// 直接导入
const name = 'dada'
let name1 = 'dada1'
var name2 = 'dada2'
export {
 name as cname
}
export default name2

import name2, {name1, name} from A复制代码

export const sqrt = Math.sqrt;
export function square(x) {
 return x * x;
}
export function dada(x,y) {
 return sqrt(square(x) + square(y));
}

import {square,da} from 'da';
console.log(square(11)); // 121
console.log();复制代码

export default function() {...}
import myFunc from 'myFunc';
export default class{...}
import MyClass from 'MyClass';
const inst = new MyClass();复制代码

require
--lib.js--
function add(x,y){
 return x + y
}
module.exports = {
 add: add,
};

--main.js--
var add = require('lib').addd;
console.log(add(1,2));复制代码

import
--lib.js--
export function add(x,y) {
 return x + y
}
--main.js--
import {add} from 'lib';
console.log(add(1,2));复制代码

--lib.js--
export const sqrt = Math.sqrt;
export function square(x) {
 return x * x;
}
export function da(x,y) {
 return sqrt(square(x)+square(y));
}
--main.js--
import {square, da} from 'lib'


--myFunc.js--
export default function() {...};
--main.js--
import myFunc from 'myFunc';
myFunc();复制代码

let da1 = [1,2,3];
console.log(da1.includes(2));复制代码

arr.find(function(item){
return item === 1;
})

arr.filter(function(item){
return item === 2;
})

Math.pow(2,3)->2**3复制代码

async function firstAsync(){
 let promise = new Promise ((resolve,reject) => {
  setTimeout(function(){
   resolve('dadaqianduan')
  },1000)
 })
 console.log(await promise)
 console.log(await Promise.resolve(1))
 console.log(2)
 return Promise.resolve(3)
}
firstAsync().then(val => {
 console.log(val)
})复制代码

let da = {
 'da': 1,
 'da2': 2
}
console.log(Object.value(da)) // [1,2]

Object.values是在对象上找到可枚举的属性的值，所以只要这个对象是可枚举的就可以复制代码

Promise.resolve().then(()=>{
 console.log(1)
}).catch(()=>{
 console.log(2)
}).then(()=>{
 console.log(3)
})复制代码

Promise.resolve().then(()=>{
 console.log(1)
 throw new Error('da')
}).catch(()=>{
 console.log(2)
}).then(()=>{
 console.log(3)
})复制代码

Promise.resolve().then(()=>{
 console.log(1)
 throw new Error('da')
}).catch(()=>{
 console.log(2)
}).catch(()=>{
 console.log(3)
})复制代码

async function fn() {
 return 1
}
(async function() {
 const a = fn() // ??
 const b = await fn() // ??
})()复制代码

console.log(100)
setTimeout( () => {
 console.log(200)
})
Promise.resolve().then( ()=> {
 console.log(300)
})
console.log(400)复制代码

async function async1() {
 console.log('async1 start')
 await async2()
 console.log('async1 end')
}

async function async2 () {
 console.log('async2')
}

console.log('script start')

setTimeout(function(){
 console.log('setTimeout')
},0)

async1()

new Promise(function (resolve){
 console.log('promise1')
 resolve()
}).then(function(){
 console.log('promise2')
})

console.log('script end')复制代码

// 加载
function  loadImg(src) {
 const p = new Promise(
  (resolve,reject) => {
   const img = document.createElement('img')
   img.onload = () =>{
    resolve(img)
   }
   img.onerror = () =>{
    const err = new Error('图片加载失败')
    reject(err)
   }
   img.src = src
  }
 )
 return p
}
const url = 'https'
const p = loadImg(url)

p.then(img =>{
 console.log(img.width)
 return img
}).then(img =>{
 console.log(img.height)
}).catch(ex => {
 console.error(ex)
})复制代码

async function async1() {
 console.log('async1 start') // 2
 await async2() // undefined
 console.log('async1 end') // 5
}
async function async2() {
 console.log('async2') // 3
}
console.log('script start') // 1
async1()
console.log('script end') // 4复制代码

function add(num) {
 return new Promise(resolve => {
  setTimeout(()=>{
   resolve(num*num)
  },1000)
 })
}
const nums = [1,2,3]
nums.forEach(async(i)=>{
 const res = await add(i)
})复制代码

function da(time) {
 return new Promise(function(resolve,reject){
  setTimeout(function(){
   resolve(time)
  },time)
 })
}
async function test() {
 let arr = [da(2000),da(1000),da(3000)]
 for await (let item of arr) {
  console.log(Date.now(), item)
 }
}复制代码

const input = {
 a: 1,
 b: 2
}
const output = {
 ...input,
 c: 3
}
console.log(output)

const input = {
 a: 1,
 b: 2,
 c: 3
}
let {a, ...rest } = input复制代码

const numbers = [1, 2, [3, 4, [5, 6]]];

// Considers default depth of 1
numbers.flat(); 

> [1, 2, 3, 4, [5, 6]]

// With depth of 2
numbers.flat(2); 

> [1, 2, 3, 4, 5, 6]

// Executes two flat operations
numbers.flat().flat(); 

> [1, 2, 3, 4, 5, 6]

// Flattens recursively until the array contains no nested arrays

numbers.flat(Infinity)
> [1, 2, 3, 4, 5, 6]复制代码

const numbers = [1, 2, 3];

numbers.map(x => [x * 2]);
> [[2], [4], [6]]

numbers.flatMap(x => [x * 2]);
> [2, 4, 6]复制代码

const records = [['name','da'], ['age', 32]];

const obj = Object.fromEntries(records);

> { name: 'da', age: 32}

Object.entries(obj);

> [['name','Mathew'], ['age', 32]];复制代码

Symbol('desc').toString();
> "Symbol(desc)"

Symbol('desc').description;  
> "desc"

Symbol('').description;      
> ""

Symbol().description;
> undefined复制代码