TypeScript Best Practices for Large-Scale Applications

TypeScript has become the default for serious JavaScript projects. Its type system catches entire categories of bugs at compile time, improves IDE support, and serves as living documentation for your codebase. But TypeScript's flexibility means you can use it well or poorly. In large-scale applications, the difference between disciplined and undisciplined TypeScript usage compounds dramatically over time. Here are the practices we follow at Born Digital.

Enable Strict Mode From Day One

Set "strict": true in your tsconfig.json from the start of every project. This enables strictNullChecks, noImplicitAny, strictFunctionTypes, and other flags that catch the most common type-related bugs. Retrofitting strict mode onto an existing codebase is painful — enabling it from the beginning is trivial.

Strict null checks alone prevent the most common runtime error in JavaScript: "Cannot read property of undefined." When the compiler forces you to handle the possibility that a value might be null or undefined, you write safer code by default.

Design Types Thoughtfully

• Avoid any: Every any type is a hole in your type safety. Use unknown when you genuinely do not know the type — it forces you to narrow before use. Reserve any for truly exceptional cases and flag them with comments.
• Prefer interfaces for objects: Interfaces are extendable, produce clearer error messages, and are the idiomatic choice for object shapes. Use type aliases for unions, intersections, and computed types.
• Use discriminated unions: Model state machines and variant types with discriminated unions. A type Result = { status: 'success'; data: T } | { status: 'error'; error: Error } pattern makes impossible states unrepresentable.
• Leverage utility types: Partial, Required, Pick, Omit, and Record reduce boilerplate and keep types DRY. Learn them — they are used constantly in production TypeScript.

Error Handling Patterns

Type your errors explicitly. Do not catch unknown exceptions and assume they are Error objects — they might be strings, numbers, or anything else JavaScript decides to throw. Use type guards to narrow caught exceptions before accessing properties. Consider the Result pattern (returning either a success or error value) instead of throwing exceptions for expected failure cases. This makes error handling explicit in your function signatures.

For API responses, define explicit response types that account for all possible states: loading, success, error, and empty. Avoid the temptation to type API responses as their happy-path shape only — real APIs return errors, partial data, and unexpected formats.

Project Structure at Scale

Organise types close to where they are used. Co-locate component props with component files rather than dumping everything into a global types/ directory. Shared types that cross module boundaries belong in a shared types package or directory, but keep the default co-located. Use barrel exports (index.ts) sparingly — they can create circular dependency issues and slow down build tools.

Common Anti-Patterns to Avoid

Avoid type assertions (as) as a substitute for proper typing — they silence the compiler rather than fixing the underlying issue. Do not over-engineer generic types when a simple concrete type suffices — readability matters more than cleverness. And resist the urge to add // @ts-ignore to silence errors; each suppressed error is a potential runtime bug waiting to surface. At Born Digital, TypeScript strict mode is non-negotiable on every project. The upfront investment in proper typing pays dividends in reduced bugs, faster onboarding, and more confident refactoring throughout the project lifecycle.