Mastering TypeScript 6.0: Desing Patterns for 2026

TypeScript has won. It is the default for web development. But most developers stop at interface props. To truly master TypeScript, you need to think in sets, distributions, and typelevel logic.

This guide covers advanced patterns used in large-scale libraries like tRPC, Zod, and Prisma.

1. The infer Keyword: Mining Type Data

The most powerful keyword in TypeScript. It allows you to extract subtypes from a larger type.

type ReturnType<T> = T extends (...args: any[]) => infer R ? R : any;

async function fetchUser() { return { id: 1, name: "Alice" }; }
type User = Awaited<ReturnType<typeof fetchUser>>;
// User is { id: number; name: string }

2. Template Literal Types (String Manipulation)

You can perform string concatenation at the type level. This is great for typings events or CSS classes.

type EventType = "click" | "hover";
type ElementType = "Button" | "Input";

type HandlerName = "on${ElementType}${Capitalize<EventType>}";
// "onButtonClick" | "onButtonHover" | "onInputClick" | "onInputHover"

3. Recursive Types (JSON & Awaitable)

Types can refer to themselves. Essential for defining JSON structures or infinitely nested arrays.

type JSONValue = string | number | boolean | null | JSONObject | JSONArray;
interface JSONObject { [key: string]: JSONValue }
interface JSONArray extends Array<JSONValue> {}

4. Opaque Types (Branded Types)

Prevent accidental assignment of similar types (e.g. UserID vs PostID) even if they are both strings.

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

type UserId = Branded<string, "UserId">;
type PostId = Branded<string, "PostId">;

const id1 = "123" as UserId;
const id2 = "456" as PostId;
// id1 = id2; // Error!

5. Mapped Types Modifiers

Remove readonly and optional flags using -.

type Mutable<T> = {
  -readonly [P in keyof T]: T[P];
}

Conclusion

Mastering these patterns allows you to build library-grade type definitions that guide developers and catch bugs at compile time. In 2025, Typescript is your documentation.