Question

So the general convention for callback functions in Node.js is to “reserve” the first parameter for an error (if one exists). For example:

callSomeBlockingFcn( function callbackWhenDone(err, result) {
  if( err ) ...
});

If you need to return more than one error–say multiple data validation errors, for example–is it considered poor form to pass an array of error objects? Example:

var callSomeBlockingFcn = function(callback) {
  // multiple errors to report back...
  callback( [ err1, err2, ...] );
}

Or is it preferable to avoid arrays and return a single object with a property referencing an array (if necessary)? Example:

var callSomeBlockingFcn = function(callback) {
  // multiple errors to report back...
  callback( { errors: [ err1, err2, ...] } );
}

Answer 1

3 years later:

Anyone that puts an array in a callback will make me mad.

The correct solution is to return an error as the first argument. If you want to return multiple errors you are probably using errors for non-exceptional cases.

In which case it should go in the “value” slot of the callback, i.e. the second argument. The first argument is for a single, unexpected operational error.

If you have multiple unexpected operational errors (unlikely) you can do something like this MultiError

Original:

I think there’s nothing wrong with returning an array of errors.

Although you could return a new custom ValidationError which has a property "messages" which is an array.

a)

function validateX(x, cb) {
  ...
  if (errorMessages) {
    return cb(errorMessages);
  }
}

b)

function ValidationError(msgs) {
  this.messages = msgs;
}

function validateX(x, cb) {
  ...
  if (errorMessages) {
    return cb(new ValidationError(errorMessages));
  }
}