新手如何快速理解js异步编程

const { EventEmitter } = require('events')
const eventEmitter = new EventEmitter()
// 订阅
eventEmitter.on("event", function(msg) {
console.log("event", msg)
})
// 发布
eventEmitter.emit("event", "Hello world")

class usrEventEmitter {
constructor () {
this.listeners = {}
}
// 订阅，callback 为每个 event 的侦听器
on(eventName, callback) {
if (!this.listeners[eventName]) this.listeners[eventName] = []
this.listeners[eventName].push(callback)
}
// 发布
emit(eventName, params) {
this.listeners[eventName].forEach(callback => {
callback(params)
})
}
// 注销
off(eventName, callback) {
const rest = this.listeners[eventName].fitler(elem => elem !== callback)
this.listeners[eventName] = rest
}
// 订阅一次
once(eventName, callback) { 
const handler = function() {
callback()
this.off(eventName, handler)
}
this.on(eventName, handler)
}
}

// 一些事件可能会被触发
eventEmitter.on
// 当它发生的时候，要这样处理
eventEmitter.emit

const promise = new Promise(function(resolve, reject) {
try {
setTimeout(() => {
resolve('hello world')
}, 500)
} catch (error) {
reject(error)
}
})
// 语义就变为先发生一些异步行为，then 我们应该这么处理
promise.then(msg => console.log(msg)).catch(error => console.log('err', error))

const { EventEmitter } = require('events')
class usrPromise extends EventEmitter {
// 构造时候执行
constructor(executor) {
super()
// 发布
const resolve = (value) => this.emit('resolve', value)
const reject = (reason) => this.emit('reject', reason)
if (executor) {
// 模拟 event loop，注此处利用 Macrotask 来模拟 Microtask
setTimeout(() => executor(resolve, reject))
}
}
then(resolveHandler, rejectHandler) {
const nextPromise = new usrPromise()
// 订阅 resolve 事件
if (resolveHandler) {
const resolve = (data) => {
const result = resolveHandler(data)
nextPromise.emit('resolve', result)
}
this.on('resolve', resolve)
}
// 订阅 reject 事件
if (rejectHandler) {
const reject = (data) => {
const result = rejectHandler(data)
nextPromise.emit('reject', result)
}
this.on('reject', reject)
} else {
this.on('reject', (data) => {
promise.emit('reject', data)
})
}
return nextPromise
}
catch(handler) {
this.on('reject', handler)
}
}

async function say(greeting) {
return new Promise(function(resolve, then) {
setTimeout(function() {
resolve(greeting)
}, 1500)
})
}
;(async function() {
let v1 = await say('Hello')
console.log(v1)
let v2 = await say('World')
console.log(v2)
})()

function async(makeGenerator) {
return function() {
const generator = makeGenerator.apply(this, arguments)
function handle({ value, done }) {
if (done === true) return Promise.resolve(value)
return Promise.resolve(value).then(
(res) => {
return handle(generator.next(res))
},
function(err) {
return handle(generator.throw(err))
}
)
}
try {
return handle(generator.next())
} catch (ex) {
return Promise.reject(ex)
}
}
}
async(function*() {
var v1 = yield say('hello')
console.log(1, v1)
var v2 = yield say('world')
console.log(2, v2)
})()