node37-promise链式编程

const fs = require('fs');
/* fs.readFile('./1.txt', 'utf8', (err, result1) => {
    console.log(result1);
    fs.readFile('./2.txt', 'utf8', (err, result2) => {
        console.log(result2);
        fs.readFile('./3.txt', 'utf8', (err, result3) => {
            console.log(result3);
        });
    });
})
 */
function p1() {
    return new Promise((resolve, reject) => {
        fs.readFile('./1.txt', 'utf8', (err, result) => {
            resolve(result);
        });
    });
}
function p2() {
    return new Promise((resolve, reject) => {
        fs.readFile('./2.txt', 'utf8', (err, result) => {
            resolve(result);
        });
    });
}
function p3() {
    return new Promise((resolve, reject) => {
        fs.readFile('./3.txt', 'utf8', (err, result) => {
            resolve(result);
        });
    });
}
p1().then((r1) => {
        console.log(r1);
        return p2();
    })
    .then((r2) => {
        console.log(r2);
        return p3();
    })
    .then((r3) => {
        console.log(r3);
    })