TypeScript — Claude Code rules

tsconfig

Assume the following flags are on (and don't disable them):

• strict: true
• noUncheckedIndexedAccess: true
• noImplicitOverride: true
• noFallthroughCasesInSwitch: true
• noUnusedLocals / noUnusedParameters: true
• verbatimModuleSyntax: true

Type discipline

• Never introduce any. If the type is unknown, type it as unknown and narrow.
• Prefer union literal types over enums (type Status = "idle" | "running" | "done"). Enums emit runtime code and have surprising structural behaviour.
• Use satisfies to keep object literals widely typed while ensuring conformance.
• as is a last-resort escape hatch. If you need it, add a comment explaining why a narrower form isn't possible.

Patterns

• Discriminated unions for state machines ({ kind: "loading" } | { kind: "loaded"; data: T }).
• Result<T, E> style returns instead of throwing for expected failures.
• Branded types (type UserId = string & { __brand: "UserId" }) for IDs that mustn't get mixed up.

Module shape

• Each module exports types alongside the values that use them.
• import type { … } for type-only imports (verbatimModuleSyntax enforces this).
• Don't re-export everything from a barrel — name the surface explicitly.