TypeScript Best Practices for Large-Scale Applications in 2026

TypeScript pays off most in large codebases where refactors and onboarding are frequent. Here are practices that keep big apps type-safe and maintainable in 2026.

## Enable Strict Mode

Use strict (and related options like strictNullChecks, noImplicitAny) in tsconfig. It surfaces bugs at compile time and prevents many runtime errors. If you are inheriting a loose config, tighten it gradually: fix the highest-traffic modules first.

## Prefer Interfaces for Public Contracts

Use interface for object shapes that are part of your public API or that may be extended (e.g. declaration merging). Use type for unions, intersections, and mapped types. Be consistent within the repo.

## Lean on Generics and Utility Types

Generics keep code DRY and type-safe. Use built-in utilities (Partial, Required, Pick, Omit, Record) before rolling your own. For function APIs, generic parameters often give better inference than broad types.

## Model Domain and API Boundaries Explicitly

Create dedicated types for API responses, DTOs, and domain entities. Do not reuse a single "any-ish" type everywhere. This makes it clear what crosses boundaries and reduces accidental coupling.

## Avoid any; Use unknown When You Must

Reserve any for rare cases (e.g. legacy integration). Prefer unknown when the type is truly dynamic and narrow with type guards or assertions only where necessary.

## Keep Types Close to Implementation

Colocate types with the code that uses them (same file or adjacent .types file). Export shared types from a dedicated module (e.g. types/index.ts) so imports stay predictable.

## Use Const Assertions and Literal Types

Use as const and literal types for configuration, enums-like sets, and discriminated unions. They give better inference and catch more mistakes.

## Document Complex Types

For non-obvious generics or conditional types, add a short comment. It helps future you and teammates. Avoid over-documenting the obvious.

Large-scale TypeScript stays manageable when the team agrees on strictness, consistent patterns, and clear boundaries. Start with strict mode and one or two of these practices; add more as the codebase grows.