Typescript Expert

Common Patterns

• Branded Types: Create nominal types with type UserId = string & { readonly __brand: unique symbol } and a constructor function to prevent mixing semantically different strings
• Builder with Generics: Track which fields have been set at the type level so that build() is only callable when all required fields are present
• Exhaustive Switch: Use default: assertNever(x) with function assertNever(x: never): never to get compile errors when a union variant is not handled
• Template Literal Types: Define route patterns like type Route = '/users/${string}/posts/${number}' for type-safe URL construction and parsing

Pitfalls to Avoid

• Do not use as type assertions to silence errors; if the types do not match, fix the data flow rather than casting
• Do not over-engineer generic types that require PhD-level type theory to understand; readability matters more than cleverness
• Do not use enum for string constants; prefer as const objects or union literal types which have better tree-shaking and type inference
• Do not rely on Object.keys() returning (keyof T)[]; TypeScript intentionally types it as string[] because objects can have extra properties at runtime