Part 21 of the series: "Extending bpmn-io Form-JS Beyond Its Limits" Every extension in this series — the evaluators, validators, providers, renderers, file handlers — ultimately gets loaded into a form instance. Something has to create that instance, configure which modules it loads, and expose the capabilities that application code needs to call. That something is CustomForm and CustomFormEditor . These two classes are thin subclasses of Form-JS's Form and FormEditor . They don't reimagine the base classes — they add exactly what the extension system needs and nothing more. The _isRendered flag so consumers know when it's safe to interact with the form. The _pendingFilesRef store so file handling works across framework boundaries. The form.rendered event so evaluators know when the DOM exists. The module list so every extension is loaded consistently. This article explains why each addition exists, why subclassing is the right approach over wrapping, and shows the complete implementation of both classes.…