TypeScript Advanced: Patterns, Pitfalls & type-fest

This skill defines the rules, conventions, and architectural decisions for writing advanced TypeScript. It is intentionally opinionated to prevent common type-level bugs and enforce patterns that produce safe, maintainable code.

For detailed API documentation of TypeScript features, use other appropriate tools (documentation lookup, web search, etc.) — this skill focuses on when, why, and how to use advanced type features correctly.

Type Safety Philosophy

any vs unknown vs never — the only rule you need

Type	Assignable from	Assignable to	Operations	Use for
any	anything	anything	all (UNSAFE)	Never in new code
unknown	anything	only unknown / any	none unnarowed	External inputs, JSON, user data
never	nothing	anything	none	Exhaustive checks, unreachable code

Rule: never use any in new code. Use unknown for external boundaries and narrow before operating. Use never for exhaustiveness and impossible states.

Prefer unions over enums

// Avoid — numeric enums are structurally assignable to number (footgun)
enum Direction {
  Up,
  Down,
  Left,
  Right,
}
function go(d: Direction) {}
go(42); // no error — TypeScript allows any number!

// Prefer — exhaustive, tree-shakeable, no runtime artifact
type Direction = "up" | "down" | "left" | "right";

String enums are safer than numeric but still carry runtime overhead and import friction. String literal unions are the default choice unless you need reverse mapping.

interface vs type — decision table

Scenario	Use	Why
Object shapes, class contracts	interface	Declaration merging, better error messages
Unions, intersections, mapped/conditional	type	Only type supports these
Third-party augmentation needed	interface	Only interfaces support declaration merging
Public API types (libraries)	interface	Consumers can augment; better display in tooltips
Internal computed types	type	More expressive, no accidental merging

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Discriminated Unions & Exhaustive Checks

The never exhaustiveness pattern

Every switch / if-else chain on a discriminated union must handle all variants. Use the never assignment to get a compile-time error when a new variant is added:

type Result<T> =
  | { status: "ok"; data: T }
  | { status: "error"; error: Error }
  | { status: "loading" };

function handle<T>(result: Result<T>): string {
  switch (result.status) {
    case "ok":
      return JSON.stringify(result.data);
    case "error":
      return result.error.message;
    case "loading":
      return "Loading...";
    default:
      const _exhaustive: never = result;
      return _exhaustive; // compile error if a variant is unhandled
  }
}

Rules for discriminated unions

• Discriminant must be a literal type — string, number, boolean literals. Wide types like string do not narrow.
• Keep the discriminant property name consistent across all members (kind, type, status).
• Avoid optional discriminants — status?: "ok" | "error" breaks narrowing.

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Branded Types — Nominal Safety in a Structural System

TypeScript is structural: UserId (a string) and OrderId (a string) are interchangeable by default. Branded types break this at the type level with zero runtime overhead.

Recommended pattern: unique symbol brand

declare const __brand: unique symbol;
type Brand<T, B> = T & { readonly [__brand]: B };

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

// Constructor = the single trust boundary, validate here
const toUserId = (id: string): UserId => id as UserId;
const toOrderId = (id: string): OrderId => id as OrderId;

function getUser(id: UserId) {
  /* ... */
}
getUser(toUserId("abc")); // ok
getUser(toOrderId("abc")); // ERROR — OrderId not assignable to UserId
getUser("abc"); // ERROR — string not assignable to UserId

When to use branded types

• IDs — UserId, OrderId, ProductId prevent cross-assignment
• Units — Meters, Feet, USD, EUR prevent arithmetic mistakes
• Validated strings — Email, URL, Slug encode that validation has happened
• Opaque tokens — JWTToken, APIKey prevent accidental logging/display

type-fest alternative

Use Tagged<T, Tag> from type-fest for multi-tag composition and metadata:

import type { Tagged, GetTagMetadata } from "type-fest";
type UserId = Tagged<string, "UserId">;
type AdminId = Tagged<UserId, "Admin">; // composable — both tags preserved

---

Modern Inference Tools

satisfies — validate without widening (TS 4.9+)

type Theme = Record<"primary" | "secondary", string | string[]>;

// Type annotation: loses specific types
const t1: Theme = { primary: "#000", secondary: ["#111", "#222"] };
t1.secondary.map((s) => s); // ERROR — string | string[] has no .map

// satisfies: validates structure, keeps specific inference
const t2 = { primary: "#000", secondary: ["#111", "#222"] } satisfies Theme;
t2.secondary.map((s) => s); // ok — inferred as string[]

Use satisfies when: you want config validation (catch typos in keys) but also need autocomplete on specific values.

const type parameters — generic literal inference (TS 5.0+)

// Without const: T = string[]
function routes<T extends string[]>(r: T): T {
  return r;
}

// With const: T = readonly ["users", "posts"]
function routes<const T extends string[]>(r: T): T {
  return r;
}
const r = routes(["users", "posts"]); // readonly ["users", "posts"]

Use const type parameters when: building registries, config factories, or any generic where preserving literal types at the call site matters.

NoInfer<T> — control inference sources (TS 5.4+)

Prevents a parameter from contributing to type inference — it reads T but doesn't influence what T becomes:

function createFSM<const TState extends string>(config: {
  states: TState[];
  initial: NoInfer<TState>; // must be from states, can't introduce new values
}) {
  /* ... */
}

createFSM({ states: ["idle", "running"], initial: "idle" }); // ok
createFSM({ states: ["idle", "running"], initial: "stopped" }); // ERROR

Use NoInfer when: a function has multiple parameters sharing a type param, and one should be constrained to what the others infer — not contribute new candidates.

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type-fest: Don't Reinvent the Wheel

type-fest provides 200+ utility types with zero runtime cost (types-only). Always check type-fest before writing a custom utility.

import type { Simplify, Merge, SetRequired, LiteralUnion } from "type-fest";

Decision table: built-in vs type-fest

Need	Built-in	type-fest
Make keys optional/required	Partial, Required	SetOptional, SetRequired (per-key)
Deep partial/readonly	—	PartialDeep, ReadonlyDeep
Merge two types (override, not intersect)	—	Merge, MergeDeep
Flatten intersection for readability	—	Simplify
String union with autocomplete + string	—	LiteralUnion
At-least/exactly-one constraint	—	RequireAtLeastOne, RequireExactlyOne
Nominal/branded types	—	Tagged, UnwrapTagged
JSON round-trip type	—	Jsonify
Strict omit (key must exist)	Omit (loose)	Except (strict)
Deep dot-path access	—	Paths, Get
Exact object (reject excess props)	—	Exact
Pick/omit by value type	—	ConditionalPick, ConditionalExcept
Package.json / tsconfig types	—	PackageJson, TsConfigJson
Case conversion for keys	—	CamelCasedProperties, etc.

Most commonly needed utilities

• Simplify<T> — flattens A & B & C into readable { ...all keys }. Use on any intersection that produces unreadable hover tooltips.
• Merge<A, B> — A & B produces never when keys conflict; Merge cleanly overrides. Use instead of & when types share key names.
• LiteralUnion<Literal, Base> — 'a' | 'b' | string kills autocomplete; LiteralUnion preserves it. Essential for extensible string APIs.
• SetRequired<T, K> / SetOptional<T, K> — toggle specific keys without maintaining duplicate interfaces.
• Jsonify<T> — models JSON.parse(JSON.stringify(x)). Catches Date → string, undefined → dropped, interface open-index issues.

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Common Pitfalls

1. any leaks silently — one any propagates through assignments, generics, and return types. A single any in a utility type makes all downstream types unsound. Use unknown + narrowing instead.

2. Excess property checks only apply to literals — assigning through a variable bypasses excess property checks entirely. Don't rely on them for runtime safety.

   interface Point {
     x: number;
     y: number;
   }
   const obj = { x: 1, y: 2, z: 3 };
   const p: Point = obj; // no error — z slips through
   

3. Distributive conditional type on never — T extends X ? A : B where T is never returns never (not B). Wrap in tuples: [T] extends [X].

4. Omit doesn't check key existence — Omit<T, "typo"> silently succeeds. Use Except from type-fest for strict key checking.

5. Type widening with let — let x = "hello" is string, not "hello". Use const, as const, or satisfies to preserve literals.

6. & intersection with conflicting keys — { a: string } & { a: number } makes a: never. Use Merge from type-fest instead.

7. Enum numeric assignability — enum Foo { A, B } allows const x: Foo = 999. Use string literal unions instead.

8. interface accidental merging — two interface User {} declarations in the same scope silently merge. Use type for internal types that should not be extended.

9. const enum under isolatedModules — esbuild, SWC, Babel all use isolatedModules. const enum in .d.ts or library code breaks these builds.

10. Forgetting readonly on array parameters — function f(arr: string[]) allows mutation. Use readonly string[] for params you don't intend to mutate.

11. Structural subtyping function params — method syntax push(x: T) is bivariant (unsound). Use function property syntax push: (x: T) => void under strictFunctionTypes for correct variance.

12. Reinventing type-fest utilities — check type-fest before writing DeepPartial, DeepReadonly, Merge, branded types, or key manipulation types. The library handles edge cases (circular refs, readonly arrays, maps/sets) that hand-rolled versions miss.

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Reference Files

Read the relevant reference file when working with a specific pattern:

File	When to read
references/conditional-types.md	infer, distributive conditionals, constraining with extends
references/mapped-types.md	Key remapping, filtering, template literal key manipulation
references/template-literals.md	String manipulation at type level, pattern matching, parsing
references/module-augmentation.md	Declaration merging, extending third-party types, global scope
references/type-fest.md	Full type-fest utility catalog by category with usage examples