Promise

回调地狱

const fs = require('fs');fs.readFile('./a.txt', 'utf-8', (err, data) => {if(err) throw err;console.log(data);fs.readFile('./b.txt', 'utf-8', (err, data) => {if(err) throw err;console.log(data);fs.readFile('./c.txt', 'utf-8', (err, data) => {if(err) throw err;console.log(data);})})
})

Promise

Promise对象可以解决回调地狱的问题

Promise可以理解为一个容器，里面可以编写异步程序的代码

基本用法

const fs = require('fs');//1、创建对象
let p = new Promise((resolve, reject) => {//把异步任务成功的结果传递给resolve函数，把失败的结果传递给reject函数fs.readFile('./a.txt', 'utf-8', (err, data) => {//读取文件的时候，没有错误err=null, 有错误err={} err ? reject(err) : resolve(data);})
});//2、获取成功失败结果
p.then(res => console.log(res), err => console.log(err));

Promise三种状态

pending fulfilled rejected

• 最初状态pending, 等待中，此时promise的结果为undefined;

• 当调用resolve(data)时， 状态变为 fulfilled

• 当调用reject(err)时， 状态变为 error

• 当达到最终的fulfilled或rejected时，promise的状态就不会再改变了（所以多次调用resolve，后面的resolve都无效，只取第一次）

  // 1、创建对象
  let p = new Promise((resolve, reject) => {
  resolve();
  resolve(123);//这次调用无效
  });

  // 2、获取成功失败结果
  p.then(res => console.log(res), err => console.log(err));

Promise同步异步?

console.log(1);//new Promise属于同步代码
new Promise((resolve, reject) => {console.log(2);resolve(3);console.log(4);
}).then(res => console.log(res), err => console.log(err));//.then这里是异步的console.log(5);// 1 2 4 5 3

then的链式调用

const fs = require('fs');let p1 = new Promise((resolve, reject) => {fs.readFile('./a.txt', 'utf-8', (err, data) => {err ? reject(err) : resolve(data);})
});let p2 = new Promise((resolve, reject) => {fs.readFile('./b.txt', 'utf-8', (err, data) => {err ? reject(err) : resolve(data);})
});let p3 = new Promise((resolve, reject) => {fs.readFile('./c.txt', 'utf-8', (err, data) => {err ? reject(err) : resolve(data);})
});//前一个then返回一个Promise对象
//后一个then可以得到前一个Promise对象的成功状态的值
p1.then(r1 => {console.log(r1); //aaareturn p2
}).then(r2 => {console.log(r2); //bbbreturn p3;
}).then(r3 => {console.log(r3); //ccc
})const fs = require('fs');function myReadFile(filename) {return p = new Promise((resolve, reject) => {fs.readFile(filename, 'utf-8', (err, data) => {err ? reject(err) : resolve(data);})})
}//前一个then返回一个Promise对象
//后一个then可以得到前一个Promise对象的成功状态的值
myReadFile('./a.txt').then(r1 => {console.log(r1); //aaareturn myReadFile('./b.txt');
}).then(r2 => {console.log(r2); //bbbreturn myReadFile('./c.txt');
}).then(r3 => {console.log(r3); //ccc
})