Promise是构造函数

原型上的方法then、catch、finally
其余都在构造函数上：all、race、reject、resolve
实例化的参数executor是一个函数
executor里的代码是同步的
executor接收2个参数resolve、reject，这2个回调函数是异步的
执行顺序同步>resolve（异步微任务）>setTimeout（异步宏任务）

Promise对象实现的 Ajax

const getJSON = function (url) {const promise = new Promise(function (resolve, reject) {const handler = function () {console.log('client实例', this)if (this.readyState !== 4) {return;}if (this.status === 200) {resolve(this.response);} else {reject(new Error(this.status));}};const client = new XMLHttpRequest();client.open("GET", url);client.onreadystatechange = handler;client.responseType = "json";client.setRequestHeader("Accept", "application/json");client.send();});return promise;
};getJSON("/posts.json").then(function (json) {console.log('Contents: ' + json);
}, function (error) {console.log('请求状态码', error); // 请求状态码 Error: 404
});

在这里插入图片描述

链式调用

每次.then返回的是一个新的Promise实例，在下一个.then接收

返回值是Promise时

const p1 = new Promise(function (resolve, reject) {console.log('p1', new Date()) // p1 Sun Jul 25 2021 15:33:01 GMT+0800 (台北标准时间)setTimeout(() => {console.log('10s后p1-resolved', new Date()) // 10s后p1-resolved Sun Jul 25 2021 15:33:11 GMT+0800 (台北标准时间)reject(new Error('fail'))}, 10000)
})const p2 = new Promise(function (resolve, reject) {console.log('p2', new Date()) // p1 Sun Jul 25 2021 15:33:01 GMT+0800 (台北标准时间)setTimeout(() => {console.log('1s后p2-resolved', new Date(), p1) // 1s后p2-resolved Sun Jul 25 2021 15:33:02 GMT+0800 (台北标准时间)resolve(p1)}, 1000)
})p2.then(result => console.log(result)).catch(error => console.log('err', error)) // err Error: fail

异步代码

分为宏任务（宏任务队列）、微任务（微任务队列）
除了resolve、reject和node的process.nextTick是微任务，其他是宏任务？
事件循环、轮询

const { log } = console
// 一开始的微任务队列 q1
Promise.resolve().then(() => {log('promise1')setTimeout(() => { // 进入q2log('st2')})
})
// 一开始的宏任务队列 q2
setTimeout(() => {log('st1')Promise.resolve().then(() => {log('promise2')})
})
// promise1
// st1
// promise2
// st2

executor报错

会吃掉状态固化以后，executor代码的错误

const p = new Promise((resolve, reject) => {resolve(1)console.log(a) // 报错 但是外部不体现
})
p.then((data) => {console.log(data)//1
})
new Promise((resolve, reject) => {resolve(2);console.log('P2同步');
}).then(r => {console.log(r);//2
});
// P2同步
// 1
// 2

const p = new Promise((resolve, reject) => {resolve(a) // 这样报错外部就会体现了// ReferenceError: a is not defined
})
p.then((data) => {console.log(data)
})

构造器上方法

参数是有iterator接口的数据结构

all

都成功，返回都成功的结果，在.then里能拿到这个数组
若有一出错，在.catch里拿到第一个失败的？

race

无论成功还是失败，返回第一个成功or失败的Promise

thenable

let obj = {then: function (resolve, reject) {resolve(1)}
}
let p = Promise.resolve(obj) // 转成promise对象
p.then((val) => {console.log(val) // 1
})

// reject怎么做

promisify

手写promisify

const fs = require('fs')
function readFile(path) {return new Promise((resolve, reject) => {fs.readFile(path, 'utf-8', (err, data) => {if (err) {reject(new Error(err))} else {resolve(data)}})})
}
readFile('first.txt').then((data) => {return readFile(data)
}).then((data) => {return readFile(data)
}).then((data) => {console.log(data)
})

使用node提供的工具类方法

const fs = require('fs')
function promisify(func) {return function (...args) {return new Promise((resolve, reject) => {func(...args, (err, data) => {if (err) {reject(new Error(err))} else {resolve(data)}})})}
}
let readFile = promisify(fs.readFile)
readFile('first.txt', 'utf-8')
.then(data =>readFile(data, 'utf-8'))
.then(data =>readFile(data, 'utf-8'))
.then(data => console.log(data))

const fs = require('fs')
const util = require('util')
let readFile = util.promisify(fs.readFile)
readFile('first.txt', 'utf-8')
.then(data =>readFile(data, 'utf-8'))
.then(data =>readFile(data, 'utf-8'))
.then(data => console.log(data))

将fs上的所有方法都promisify

function promisifyAll(obj) {for (let [key, fn] of Object.entries(obj)) {if (typeof fn === 'function') {obj[key + 'Async'] = promisify(fn)}}
}
promisifyAll(fs)
function promisify(func) {return function (...args) {return new Promise((resolve, reject) => {func(...args, (err, data) => {if (err) {reject(new Error(err))} else {resolve(data)}})})}
}
let readFile = fs.readFileAsync