Custom Error Messages with Mongoose

Is unique parameter not supported for custom messages?

Uniqueness in Mongoose is not a validation parameter (like required); it tells Mongoose to create a unique index in MongoDB for that field.

The uniqueness constraint is handled entirely in the MongoDB server. When you add a document with a duplicate key, the MongoDB server will return the error that you are showing (E11000...).

You have to handle these errors yourself if you want to create custom error messages. The Mongoose documentation ("Error Handling Middleware") provides you with an example on how to create custom error handling:

emailVerificationTokenSchema.post('save', function(error, doc, next) {
  if (error.name === 'MongoError' && error.code === 11000) {
    next(new Error('email must be unique'));
  } else {
    next(error);
  }
});

(although this doesn't provide you with the specific field for which the uniqueness constraint failed)

It is not directly possible as you tried it but you may want to have a look at mongoose-unique-validator which allows for custom error message if uniqueness would be violated.

In particular the section about custom errors should be of interest to you.

To get your desired

"email must be unique"

it would look similar to this

var uniqueValidator = require('mongoose-unique-validator');
...
emailVerificationTokenSchema.plugin(uniqueValidator, { message: '{PATH} must be unique' });

verificationToken.save( function (err) {
    if (err) {
      return res.status(400).send({
          message: (err.name === 'MongoError' && err.code === 11000) ? 'Email already exists !' : errorHandler.getErrorMessage(err)
      });
    }
    else {
      return console.log('No Error');
    }
  });