cb.js

callbackMethod(input, function myHandler(error, result) {
  if (error)
  {
    // check for error, since it's in my face here as first argument
    return;
  }
    
  // do other things
});




// vs



promiseMethod(input).then(function halfOfMyHandler(result) {
  // looks like I forgot to check for errors at all
  // just do things
});


// vs


promiseMethod(input).then(function halfOfMyHandler(result) {
  // just do things
}).catch(function anotherHalfOfMyHandler(error) {
  // so why do I need to write two functions? o_0
});

From: http://paste.ubuntu.com/23427957/

class Service {
  businessLogic(result) {
    //do something
  }
  
  errorHandler(error) {
    if (error) {
      return;
    }
  }
}

var sevice = new Srvice();

promiseMethod(input).then(service.businessLogic}).catch(service.errorHandler);

//vs

promiseMethod(input).then(result => {
  // just do things
}).catch(error => {
  // so why do I need to write two functions? o_0
});

In real life app, you'd have more than one result handler and more than one error handler, and it will be more cognitive load try to keep track if you used proper error handler for the given result handler.

I personally, not a fan of separating error handling from "business logic", since it's not pure business logic at this level (where your code touches some kind of I/O) and usually you'd keep your "true business logic" in a separate module.

var myLogic = require('./lib/my_logic.js');

callbackMethod(input, function myHandler(error, result) {
  if (error)
  {
    // check for error, since it's in my face here as first argument
    return;
  }
    
 myLogic.doThings(result.data);
});