Best way to handle the error in async node

Question

To catch errors, I have written if-else blocks in every function which looks bad. Please suggest a better way to handle errors in async node.

 async.waterfall([
 function(callback){
 fnOne.GetOne(req, res,function(err,result) { 
 if(err){
 console.error("Controller : fnOne",err);
 callback(err,null);
 }
 else{
 var fnOne = result; 
 callback(null, fnOne);
 }
 }) 
 },
 function(fnOne, callback){
 fnTwo.two(fnOne,function(err,result) {
 if(err) {
 console.error(err);
 callback(err,null);
 }
 else{
 callback(null, context);
 }
 }) 
 }
 ], function (err, result) { 
 if(err){
 console.error("Controller waterfall Error" , err);
 res.send("Error in serving request.");
 }
 });

Answer 1

Check out promises. They are not (yet) available natively in Node.js, but you can use Q library.

Promises allow you to make your control flow explicit with methods like catch and finally instead of hiding it in callbacks.

Also you won't need async with promises.

Q provides a convenience method to "convert" a function that accepts a callback to a promise-returning function.

I don't understand what fnOne or fnTwo is your code (seemingly objects and not functions), so I can't really translate your example, but it would look similar to this:

var Q = require('q');
// Convert our functions to promise-returning functions
var getOne = Q.nbind(fnOne.GetOne, fnOne),
 getTwo = Q.nbind(fnTwo.two, fnTwo) ;
getOne(req, res)
 .then(function (result) {
 return getTwo(result);
 })
 .then(function (result) {
 // do something useful with final result
 })
 .catch(function (err) {
 console.error("Three was an error", err); 
 res.send("Error in serving request.");
 })
 .done();