typescript-best-practices

TypeScript best practices

A skill named in bold or in backticks, or run as /<name>, is the file ../<name>/SKILL.md beside this skill's directory, and a principle named without its prefix is ../principle-<name>/SKILL.md. Read the file and follow it. Most pstack skills are user-only, so your skill tool cannot load them.

Apply the type-system-discipline principle skill first.

Rule	Summary
Discriminated unions	Model variants with a kind literal discriminant so impossible states can't be represented. No optional-field bags.
Branded types	Brand primitives with & { readonly __brand: "X" } so they can't be mixed up. Validate once at the boundary. A brand comes from a verifying type guard (input is AgentId), never from as.
Constructive modeling	Build the shape so the illegal value can't be constructed. [T, ...T[]] for non-empty, [T, T][] for even length, start plus duration for a range. Not a runtime guard, not a wish for refinement types.
Simplest total type	Keep T[] while every operation on it stays total. Strengthen to NonEmpty<T> only where the loose type forces !, a cast, or a "should never happen" throw.
unknown over any	External data is unknown.
Schemas before guards	Before hand-writing a property-by-property type guard, use the repository's runtime schema library and infer the type from the schema, such as z.infer.
No as casts	Every as is a runtime crash waiting. Cast only after validation.
Narrowing hierarchy	Discriminant switch > in operator > typeof/instanceof > user-defined type guard > as.
Type guards	Must verify the claim. A lying guard is worse than as because the bug hides behind a name that says it's safe. Name them isX or hasX.
Exhaustiveness	Inline const _exhaustive: never = x; in default arms so the compiler errors when a new variant is added.
satisfies over as	Validates the value without widening literal types.
Boundary validation	Parse where data crosses in, into a named domain type. Record<string, unknown> (however spelled) stops at that parse. Trust types inside. See the boundary-discipline principle skill.
Schema-derived types	Reach for Pick/Omit/Parameters/ReturnType/Awaited/typeof before declaring a new interface.
Object args	Pass objects, not positional, so argument order is self-documenting. Skip on hot paths (per-frame render, tokenizers, parsers).
Real tests	Don't mock what you can run. Prefer the framework's real test primitives with leak/disposable checks, and verify UI in a running build. Mock only what you can't run locally.
Structured telemetry	Prefer structured logger diagnostics with enough context to debug from an id. No console.log in shipped code.

Examples: references/patterns.md.