JavaScript异步编程解决方案Promise、Generator、Async

下面通过按顺序读取本地文件data/1.txt-->data/2.txt-->data/3.txt三个问题来介绍三者的使用方式和差别

 1，使用Promise实现

const fs = require('fs');

function readFile(filename) {
    return new Promise((resolve, reject) => {
        fs.readFile(filename, (err, data) => {
            if (err) {
                reject(err);
            } else {
                resolve(data);
            }
        });

    });
}

readFile('data/1.txt').then(res => {
    console.log(res.toString());
    return readFile('data/2.txt');
}).then(res => {
    console.log(res.toString());
    return readFile('data/3.txt');
}).then(res => {
    console.log(res.toString());
});

 2，使用Generator的方式实现

const fs = require('fs');

function readfile(pathname) {
    return new Promise((resolve, reject) => {
        fs.readFile(pathname, (err, data) => {
            if (err) {
                reject(err);
            } else {
                resolve(data);
            }
        });

    });
}

function* gen() {
    yield readfile('data/1.txt');
    yield readfile('data/2.txt');
    yield readfile('data/3.txt');
}
let g1 = gen();
g1.next().value.then(res => {
    console.log(res.toString());
    return g1.next().value;
}).then(res => {
    console.log(res.toString());
    return g1.next().value;
}).then(res => {
    console.log(res.toString());
});

3，使用Async的方式实现

const fs = require('fs');

function readfile(pathname) {
    return new Promise((resolve, reject) => {
        fs.readFile(pathname, (err, data) => {
            if (err) {
                reject(err);
            } else {
                resolve(data);
            }
        });

    });
}
async function asy() {
    let a = await readfile('data/1.txt');
    let b = await readfile('data/2.txt');
    let c = await readfile('data/3.txt');
    console.log(a.toString(), b.toString(), c.toString());
}
asy();

async特点

1,await要放在async函数体中

2,比generator更加语义化

3,await后面可以是promise对象，也可以是数字，字符串和布尔值

4,只要await语句后面的Promise状态变成reject,整个async函数会中断执行

如何解决async函数里面抛出错误，影响后续执行

async function asy() {
 try{
    let a = await readfile('data/1.txt');
    let b = await readfile('data/2.txt');
    let c = await readfile('data/3.txt');
    console.log(a.toString(), b.toString(), c.toString());
  }catch(e){} 
} 
asy();