When we have been creating, editing, and saving data in our project, we have been using a callback function containing two parameters: an error object and the returned data object. So far we have generally been ignoring the error object and just been saving the data, for example:

user.save(function (err, user) {
  if(err){
    console.log(err)
  } else { ...

It is in the err object that we receive the Mongoose validation errors. The error object will contain a top-level message and name, and a collection of specific errors. Each of these errors give an individual message, name, path, and type object. A typical validation failure looks like the following when sent to the console:

{ message: 'Validation failed',
  name: 'ValidationError',
  errors: 
   { email: 
      { message: 'Validator "required" failed for path email',
        name: 'ValidatorError',
        path: 'email',
        type: 'required' },
     name: 
      { message: 'Validator "required" failed for path name',
        name: 'ValidatorError',
        path: 'name',
        type: 'required' } } }

Knowing the structure of the returned error object helps us to identify the exact reason for the validation failure and send a relevant error message to the user.

To output just a summary to the console, you could loop through the error objects with the following code snippet:

if(err){
  Object.keys(err.errors).forEach(function(key) {
    var message = err.errors[key].message;
    console.log('Validation error for "%s": %s', key, message);
  });
}

This would result in a much more manageable output, which is as follows:

Validation error for "email": Validator "required" failed for path email
Validation error for "name": Validator "required" failed for path name