TypeScript Best Practices for Large Codebases

TypeScript adoption accelerates in 2025, with 85% of new JavaScript projects choosing TypeScript. However, TypeScript's power creates challenges at scale. At Acceli, we maintain TypeScript codebases exceeding 200,000 lines. This guide distills patterns that scale from small projects to enterprise applications, focusing on decisions that compound over time.

Strict Mode Is Non-Negotiable

Enable strict mode from day one: strict: true, noUncheckedIndexedAccess: true, exactOptionalPropertyTypes: true. One client migration from loose to strict typing took 4 weeks fixing 2,800+ errors in 80,000 lines, but resulted in 60% reduction in null/undefined production bugs. Starting strict is vastly cheaper than retrofitting. For legacy code, migrate incrementally using directory-based strictness and track progress with type-coverage tool.

Type Design Patterns

Use discriminated unions for state management instead of boolean flags—prevents impossible states. Implement branded types for domain primitives (UserId, ProductId) to prevent mixing incompatible IDs. Use template literal types for type-safe API endpoints. These patterns prevented numerous production bugs across our client projects while improving code documentation.

Generated Types

Generate TypeScript types from external sources: OpenAPI specs (openapi-typescript), database schemas (Prisma, Kysely), and GraphQL schemas (GraphQL Code Generator). This ensures frontend-backend type consistency and catches breaking changes in CI builds. One client eliminated 100% of type mismatches between frontend and backend using generated types.

Performance Optimization

Enable incremental compilation (reduced build times 180s to 35s for 150K-line codebase). Use type-only imports to reduce bundle size. Implement project references for monorepos to enable incremental builds (one client: 140s to 25s). Use path mapping to avoid relative import hell and establish clear module boundaries.

Conclusion

TypeScript's value in large codebases comes from disciplined patterns applied consistently. Strict mode, discriminated unions, branded types, and generated types compound over time, dramatically improving code quality. Start with strict mode and establish patterns early. For existing projects, migrate incrementally but deliberately with clear goals and type coverage tracking.