First, a little backstory as to why I feel this would be a valuable addition to CBValidation:

I've found that most complex entity objects are composed of child objects (composed properties). Validating the parent (aggregate root) object often involves ensuring the children are also valid. The best way to do this with CBValidation is to use a UDF or method validator within the parent to validate() the child objects, and if they pass their own constraints, bubble the success up to the parent like this:

// root object constraints
this.constraints = {
    "childObject": { udf: function( value, target ) {
            // validate the child object using its own constraints
            var validationResult = validationManager.validate( value );
            return !validationResult.hasErrors();
    } };
}

However, providing meaningful error messages when performing complex validations via UDFs or methods is extremely difficult, if not impossible.

We can allow users to have more control over their error messages by leveraging the existing error metadata and the custom message replacement system already in place in CBValidation. By simply passing along the errorMetadata struct as an additional argument to the UDF (or method), we give developers the ability and flexibility to customize their error messages to their heart's content.

Here's an example of how you might do that with a UDF validator with this new feature in place:

// root object constraints
this.constraints = {
    "childObject": { udf: function( value, target, metadata ) {
            // ... perform some complex validation
            // create a custom message and store it in the metadata.
            metadata[ "customMessage" ] = "This is a custom error message from within the udf";
            return false; 
        },
        // the message replacement system will look for a matching metadata key and replace it with the message from the UDF.
        udfMessage: "{customMessage}" 
    };
}

Hopefully, you will find this new feature a useful addition. Additionally, the feature shouldn't negatively impact anyone currently using CBValidation since we are just adding an extra argument to the UDF/method validators. In other words, this shouldn't introduce any breaking changes.

Please let me know if you do merge this change into production and I'll work on some documentation changes and examples to showcase the new feature.