TypeScript is widely used but often underused. Most teams enable strict mode and add types to their functions — but stop before reaching the patterns that genuinely prevent bugs. Here's what we apply on every project.
Branded types for domain primitives
A user ID and an order ID are both strings. Without branding, TypeScript lets you pass one where the other is expected. Branded types fix this: type UserId = string & { readonly __brand: 'UserId' }. Now getUserById(orderId) is a compile error, not a runtime bug. We brand every domain ID, currency amount, and measurement unit.
Discriminated unions for state machines
Instead of an object with optional fields and a status string, model state explicitly. A payment is either { status: 'pending' } or { status: 'completed', transactionId: string } or { status: 'failed', error: string }. TypeScript narrows the type in each branch of a switch — accessing transactionId on a pending payment is a compile error.
Template literal types for string contracts
Route definitions, event names, CSS class patterns — anywhere you have structured strings. type Route = `/api/${string}` or type EventName = `${'user' | 'order'}.${'created' | 'updated' | 'deleted'}`. Mistyped event names become compile errors instead of silent bugs.
Infer types from schemas, not the other way
Define your validation schema once (with Zod) and derive the TypeScript type from it. type User = z.infer<typeof UserSchema>. This ensures your type and your runtime validation are always in sync. There's one source of truth — the schema.
Const assertions for exhaustive handling
Use const assertions with union types and the never trick for exhaustive switch statements. If you add a new status to an enum and forget to handle it, you get a compile error — not a silent runtime gap.
Conditional types for API design
Return types that depend on input types. A fetch function that returns T if throwOnError is false and T | never if true. A function that returns a different shape based on which options you pass. Conditional types make function overloads type-safe without duplication.