promise解决异步操作

const fs=require("fs");

fs.readFile('./resource/为学.md',(err,data1)=>{
    fs.readFile('./resource/插秧诗.md',(err,data2)=>{
        fs.readFile('./resource/观书有感.md',(err,data3)=>{
            let result=data1+'\r\n'+data2+'\r\n'+data3;
        })
    })
})

---------------

const p=new promise((resolve,reject)=>{
    fs.readFile("./resource/为学.md",(err,data)=>{
        resolve(data);
    })
})

p.then(value=>{
    return new promise((resolve,reject)=>{
        fs.readFile("./resource/插秧诗.md",(err,data)=>{
            resolve([value,data]);
        })
    })
}).then(value=>{
    return new promise((resolve,reject)=>{
        fs.readFile("./resource/观书有感.md",(err,data)=>{
            value.push(data);
            resolve(value);
        })
    })
}).then(value=>{
    console.log(value.join('\r\n'));
})