ES6中的Promise深入学习

//1.创建一个Promise实例
let promise = new Promise(function (resolve, reject) {
      //2.执行异步操作任务
      setTimeout(()=>{
            const time = Date.now();//获取当前的时间，是偶数就表示成功，否则代表失败
            
            if(time%2 == 0){
            //3.1如果成功了，调用resolve(value)
                  resolve("异步操作成功"+time);
            }else{
            //3.2如果失败了，调用reject(error)
                  reject("异步操作失败"+time)
            }
      },1000)
})

promise.then(
    (value)=>{//接收得到成功的value数据
        console.log("成功的回调",value)
    },
    (error)=>{//接收得到失败的error数据
        console.log("失败的回调",error)
    }
);

//成功的回调函数
function successCallback(value){
    console.log("音频文件创建成功"+value)
}
//失败的回调函数
function failureCallback(error){
    console.log("音频文件创建失败"+error)
}
//使用纯回调函数
createAudioFileAsync(audioSettings,successCallback,failureCallback);

const promise = createAudioFileAsync(audioSettings);//先启动异步任务
//可以在异步任务完成之后再指定回调函数
setTimeout(()=>{
    promise.then(successCallback,failureCallback)
},3000)

const fs = require("fs");
// 读a文件
fs.readFile(`${__dirname}/etc/a.txt`, 'utf-8', (err, data) => {
    console.log(data);
    fs.readFile(`${__dirname}/etc/b.txt`, 'utf-8', (err, data) => {
        console.log(data);
        fs.readFile(`${__dirname}/etc/c.txt`, 'utf-8', (err, data) => {
            console.log(data);
        })
    })
})

getFile("/a.txt").then((value)=>{
    console.log(value)//promise中resolve函数传递过来的value值
    return 5;
}).then((value)=>{
    console.log(value)//5
}).then((value)=>{
    console.log(value)//undefined
})

getFile("/a.txt").then((value)=>{
    getFile(value.url);
}).then(value=>{
    console.log("getFile(value.url)成功后resolve的参数:"+value)
})

function getReadfilePromise(filename) {
    return new Promise((resolve, reject) => {
        fs.readFile(`${__dirname}/etc/${filename}.txt`, 'utf-8', (err, data) => {
            if (err) {
                reject(err);
            } else {
                resolve(data);
            }
        })
    })
}

// 解决回调地狱
// 如果这个then方法，它能够返回一个promise对象，那就可以继续点出then方法,这样就能依次读取a、b、c三个文件。
getReadfilePromise("a").then((res) => {
    console.log(res);
    return getReadfilePromise("b");
}).then((res) => {
    console.log(res);
    return getReadfilePromise("c");
}).then((res) => {
    console.log(res);
})

//假设p是一个promise实例
p
.then((value)=>console.log("resolved成功后的"+value))
.catch((error)=>console.log("reject失败后的"+error));
//等同于
p
.then(value)=>console.log("resolved成功后的"+value))
.then(null,(error)=>console.log("reject失败后的"+error));

let p = new Promise((resolve,reject)=>{
    throw new Error('test');
})
p.catch(error=>{
    console.log(error)//Error:test
})

let p = new Promise((resolve,reject)=>{
    resolve("ok");
    throw new Error('test');
})
p
.then(value=>{console.log(value)})
.catch(error=>{console.log(error)})//ok

//第一种写法

const getDataPromise = connectDatabase();//假设这个方法返回一个promise对象

const booksPromise = getDataPromise.then(findBooks);
const usersPromise = getDataPromise.then(getUsers);

Promise.all([booksPromise,usersPromise])
.then([books,user]=>{
    pickRecomment(books,user)
})

const p1 = new Promise((resolve, reject) => {
  resolve('hello');
})
.then(result => result)
.catch(e => e);

const p2 = new Promise((resolve, reject) => {
  throw new Error('报错了');
})
.then(result => result)
.catch(e => e);

Promise.all([p1, p2])
.then(result => console.log(result))
.catch(e => console.log(e));
// ["hello", Error: 报错了]

const p1 = new Promise((resolve, reject) => {
  resolve('hello');
})
.then(result => result);

const p2 = new Promise((resolve, reject) => {
  throw new Error('报错了');
})
.then(result => result);

Promise.all([p1, p2])
.then(result => console.log(result))
.catch(e => console.log(e));
// Error: 报错了

const p = Promise.race([
  fetch('/resource-that-may-take-a-while'),
  new Promise(function (resolve, reject) {
    setTimeout(() => reject(new Error('request timeout')), 5000)
  })
]);

p
.then(value=>{console.log(value)})
.catch(value=>{console.log(error)});

promise
.then(result => {···})
.catch(error => {···})
.finally(() => {···});

// resolve 的值是 undefined
Promise.resolve(2).then(() => {}, () => {})

// resolve 的值是 2
Promise.resolve(2).finally(() => {})

// reject 的值是 undefined
Promise.reject(3).then(() => {}, () => {})

// reject 的值是 3
Promise.reject(3).finally(() => {})

Promise.resolve('foo')
// 等价于
new Promise(resolve => resolve('foo'))

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

new Promise((resolve,reject)=>{
      //同步回调
      console.log('hello');//Promise一旦建立就会立即执行，无法中途取消
      resolve()
}).then(()=>{
      console.log("two");
})

console.log('one')
//hello
//one
//two
//three

new Promise((resolve,reject)=>{
    resolve(1);
    console.log("2");
}).then(value=>{
    console.log(value);
})
//2
//1

new Promise((resolve, reject) => {
            setTimeout(() => {
                console.log("执行任务1(异步)");
                resolve(1)
            }, 1000)
        }).then(value => {
            console.log("任务1的结果:" + value);
            console.log("执行任务2(同步)");
            return 2;
        }).then(value => {
            console.log("任务2的结果:" + value);
            return new Promise((resolve, reject) => {
                setTimeout(() => {
                    console.log("执行任务3(异步)");
                    resolve(3);
                }, 1000)
            })
        }).then(value => {
            console.log("任务3的结果:" + value);
        })
//执行任务1(异步)
//任务1的结果:1
//执行任务2(同步)
//任务2的结果:2
//执行任务3(异步)
//任务3的结果:3