Base Doc class that all business models extend. Implements a complete lifecycle (init, validate, beforeSubmit, afterSubmit, beforeCancel, afterCancel), formula-based computed fields with dependency tracking, validation rules, hidden/readOnly logic, and sync to/from the database. This is a self-contained ORM pattern that could be extracted (with its abstract database layer) into any TypeScript project needing reactive, schema-driven data models. It demonstrates non-obvious skill in managing state, reactivity (using Vue's reactive()), and formula sequencing.

Implements double-entry bookkeeping posting for any transactional document (Invoice, Payment, JournalEntry). Manages debit/credit entries with round-off adjustment, account validation, and posting reversals. This is a clean, isolated implementation of a fundamental accounting pattern that could be reused in any financial application. It abstracts ledger entry creation from the specific document type, making it reusable with minimal adaptation.

Implements FIFO (First In, First Out) and moving average inventory valuation methods as a generic queue. It handles stock additions, removals, and returns with correct cost allocation. This is a standalone module with no external dependencies on the rest of the application (only uses Money types). It could be extracted and used in any inventory management system. Demonstrates careful handling of edge cases (returns, partial consumption) and clear algorithmic structure.

Extends the base Party model for Indian GST compliance by adding fields (GSTIN, PAN, etc.), custom validations, and filters. This demonstrates a clean pattern for regional business logic extensions: override specific model methods and schema fields without modifying the base model. The same pattern is used for Address and other regional models, making it easy to add support for new countries. This is a reusable approach for any multi-tenant ERP system needing regional variations.