Promises Example (uses bluebird)

callback-example.js

var Promise = require('bluebird')
var fs = require('fs');
Promise.promisifyAll(fs);

fs.readFile("bb", function(err, data) {
    if (err) return next(err);
    fs.writeFile("bba", data, function() {
        console.log("File written");
    });
});

promise-chain-example.js

var Promise = require('bluebird')
var fs = require('fs');
Promise.promisifyAll(fs);

fs.readFileAsync("bb").then(function(data) {
    return fs.writeFileAsync("bba", data);
}).then(function() {
    return "bba";
}).then(function(name) {
    console.log("File " + name + " written");
}).
catch (function(e) {
    console.error("unable to read file")
});

promise-passing-example.js

var Promise = require('bluebird')
var fs = require('fs');
Promise.promisifyAll(fs);

var promise = fs.readFileAsync("bb").then(function(data) {
    return fs.writeFileAsync("bba", data);
}).then(function() {
    return "bba";
}).
catch (function(e) {
    console.error("unable to read file")
});
// in another layer
promise.then(function(name) {
    console.log("File " + name + " written");
});