Promisejs.org Website Review

Consider the following synchronous Ja*aScript function to read a file and parse it as JSON. It is simple and easy to read, but you wouldn't want to use it in most applications as it is blocking. This means that while you are reading the file from disk (a slow operation) nothing function readJSONSync(filename) {   return JSON.parse(fs.readFileSync(filename, 'utf8')); To make our application performant and responsive, we need to make all the operations that involve IO be asynchronous. The simplest way to do this would be to use a callback. However, a naive implementation will probably go wrong: function readJSON(filename, callback){   fs.readFile(filename, 'utf8', function (err, res){     if (err) return callback(err);     callback(null, JSON.parse(res)); We need to handle errors thrown by JSON.parse but we also need to be careful not to handle errors thrown by the callback function. By the time we've done all of this our code is a mess of error handling: function readJSON(filename, callback){   fs.readFile(filename, 'utf8', function (err, res){       res = JSON.parse(res);     } catch (ex) { Despite all this mess of error handling code, we are still left with the problem of the extra callback parameter hanging around. Promises help you naturally handle errors, and write cleaner code by not having callback parameters, and without modifying the underlying architecture The core idea behind promises is that a promise represents the result of an asynchronous operation. A promise is in one of three different Once a promise is fulfilled or rejected, it is immutable (i.e. it can never change again). Once all of the APIs return promises, it should be relatively rare that you need to construct one by hand. In the meantime, we need a way function readFile(filename, enc){   return new Promise(function (fulfill, reject){     fs.readFile(filename, enc, function (err, res){       if (err) reject(err);