typescript-strict-patterns

TypeScript Strict Patterns

Project setup — read project-setup.md when bootstrapping a new project or changing tsconfig / ts-reset / type-fest. ESLint baseline — read eslint.config.mjs when adding or tweaking lint rules.

Discriminated Unions + Exhaustive Checking

Model variants as discriminated unions — never bags of optional properties:

// GOOD — each variant carries exactly its data
type Result =
  | { status: "ok"; data: string }
  | { status: "error"; message: string };

// Exhaustive check helper — will fail to compile if a variant is missed
function assertNever(x: never): never {
  throw new Error(`Unexpected: ${JSON.stringify(x)}`);
}

function handle(r: Result) {
  switch (r.status) {
    case "ok": return r.data;
    case "error": return r.message;
    default: assertNever(r); // compile error if a case is missing
  }
}

Use satisfies never or the assertNever helper at the default: branch. ESLint's switch-exhaustiveness-check enforces this at lint time.

Branded Types

Prevent accidental interchange of structurally identical types with a brand:

type Brand<T, B extends string> = T & { readonly __brand: B };

type UserId = Brand<string, "UserId">;
type OrderId = Brand<string, "OrderId">;

function getUser(id: UserId) { /* ... */ }

const uid = "abc" as UserId; // cast once at the boundary
getUser(uid);    // OK
getUser("abc");  // compile error — plain string is not UserId

Brand at system boundaries (API response parsing, DB reads). Internal code then carries the brand without further casts.

Template Literal Types

Enforce string formats at the type level:

type HexColor = `#${string}`;
type Route = `/${string}`;
type EventName = `on${Capitalize<string>}`;

function setColor(c: HexColor) { /* ... */ }
setColor("#ff0000"); // OK
setColor("red");     // compile error

Useful for config keys, route paths, and event names where runtime validation is overkill but typos are common.