promise

var fs = require('fs');

var a = new Promise(function(resolve, reject) {
    fs.readFile('./1.txt', 'utf-8', function(err, data) {
        if (err) {
            return reject(err);
        }
        resolve(data);
    });
});
var b = new Promise(function(resolve, reject) {
    fs.readFile('./2.txt', 'utf-8', function(err, data) {
        if (err) {
            return reject(err);
        }
        resolve(data);
    });
});
a.then(function(res1) {
    console.log('res1');
    console.log(res1);
    b.then(function(res2) {
        console.log('res1 + res2');
        console.log(res1 + res2);
    });
});
// b.then(function(result){
//     console.log(result);
// });

fs.readFile('./1.txt', 'utf-8', function(err, data1) {
    if (err) {
        return reject(err);
    } else {
        console.log('data1');
        console.log(data1);
        fs.readFile('./2.txt', 'utf-8', function(err, data2) {
            if (err) {
                return reject(err);
            } else {
                console.log('data1 + data2');
                console.log(data1 + data2);
            }
        });
    }
});