The paper presents a visual, imperative language for the specification of constraints corresponding to domain-specific modeling languages (DSML) in the new WebGME modeling environment. The language is based on the visual notation introduced by Scratch. The novel feature of the approach is that the constraint language is just another DSML defined through UML class diagram-based metamodels in WebGME. As such, it is easily extensible via metamodel inheritance. The visual constraint programs are automatically translated into asynchronous JavaScript code that utilizes the native WebGME APIs for evaluation.