promise.js

var Promise = require('bluebird');
 // Converts all function of 'fs' into promises.
var fs = Promise.promisifyAll(require('fs'));

fs.readFileAsync('file.js','utf8')
// 'then' when result comes.
.then(function(data) {
  console.log(data);
})
//'catch' when error comes.
.catch(function(err) {
  console.log(err);
});