const fs = require('fs');

//简单封装  fs封装成一个promise
const readFile = function (fileName){
    return new Promise((resolve, reject) =>{
        fs.readFile(fileName, (err, data) =>{
            if(err) reject(err);
            resolve(data);
        });
    });
}


//promise
readFile('data/a.txt').then(res=>{
    console.log(res.toString());
    return readFile('data/b.txt');
}).then(res=>{
    console.log(res.toString());
    return readFile('data/c.txt');
}).then(res=>{
    console.log(res.toString());
})