A PEG-based parser (generated from grammar.pegjs) that parses survey expressions into an AST. These expressions power conditional visibility, calculated values, validation rules, and triggers. The parser is hand-tuned for survey-specific syntax (e.g., curly-brace variable references) and is a well-crafted, isolated component demonstrating non-trivial compiler design skills. Could be reused or studied for building custom expression languages.

A generic drag-and-drop engine that abstracts the complexities of DOM-based drag operations. It provides shortcut ghost rendering, drop-target detection via DOM paths, and event hooks (onDragStart, onDrop, onDragEnd) that decouple the physics from application logic. The engine is framework-agnostic and reusable for any drag-drop scenario (ranking, reordering, matrix rows). Demonstrates careful separation of concerns and non-trivial cursor/touch handling.

A modal popup manager that manages a stack of modal layers, implements focus trapping (keyboard navigation within the modal), and coordinates animations and overlays. It is independent of rendering framework and can be used in any web application. The implementation demonstrates professional-grade accessibility patterns (ARIA attributes, focus restoration) and robust state management for overlapping modals.

Defines the ILintRule interface and a registry for survey JSON linting rules. Each rule (e.g., duplicate names, unknown references, expression type mismatch) is a separate class implementing `run()`. The framework supports severity levels, suppression (via `survey.lintIgnore`), and generates structured results. This pattern is highly reusable for any JSON-validation tool and demonstrates clean separation of rule logic from the lint runner.