writing-idiomatic-typescript

Writing idiomatic TypeScript

The point of TypeScript is to make illegal states unrepresentable and let the compiler catch mistakes. Idiomatic TS works with the type system instead of around it. Assume strict: true.

Model the domain in types

• Discriminated unions for "one of N shapes" ({ kind: "ok"; value } | { kind: "err"; error }), then switch on the tag — the compiler checks exhaustiveness.
• type aliases for unions/functions/mapped types; interface for object shapes that may be extended or merged. Be consistent.
• Prefer union string literals (type Mode = "r" | "w") over enum.
• Mark data readonly where it shouldn't mutate; use as const for literal tuples/objects.
• Reach for utility types (Pick, Omit, Partial, Record, ReturnType) before restating shapes.

Stay sound

• unknown, not any, at boundaries (JSON, catch) — then narrow with type guards.
• Avoid as casts; when unavoidable, cast to the narrowest type and comment why. Never double-cast through unknown to silence an error.
• Avoid the non-null assertion !; narrow instead (if (x == null) return).
• Use satisfies to check a value against a type without widening it.
• Type guards (x is T) and in/typeof/instanceof narrowing over manual casts.

Functions & async

• Let inference do the work for locals; annotate parameters and public return types.
• async/await over raw .then chains; never leave a promise unawaited (handle or void it).
• Prefer immutable transforms (map/filter/reduce) over mutation when it reads clearly.

Anti-patterns to flag

• any (especially implicit), unchecked as, !, @ts-ignore without a justification.
• enum where a string-literal union would do; Function/Object/{} as types.
• Optional-everything interfaces that don't reflect real invariants.

American English in identifiers and comments.