TypeScript Mastery

Description

Advanced TypeScript patterns for professional-grade applications. Covers branded types for nominal typing, discriminated unions for state machines, template literal types for DSLs, generics with constraints, utility type composition, custom type guards, and performance-tuned tsconfig.json settings. Also includes a practical incremental migration strategy from JavaScript. TypeScript is the #1 language on GitHub in 2026 with 80%+ adoption in new projects.

Usage

Install this skill to get advanced TypeScript patterns including:

• Branded types to prevent mixing structurally identical types (UserId vs OrderId)
• Discriminated unions with exhaustiveness checking for state machines
• Template literal types for type-safe API routes and event names
• tsconfig.json settings that reduce compilation time by 30-50%
• Incremental migration strategy from JavaScript to TypeScript

When working on TypeScript projects, this skill provides context for:

• Designing type-safe APIs using generics and constraints
• Replacing any with unknown + type narrowing
• Using as const instead of enums (better tree-shaking)
• Validating runtime data with Zod instead of type assertions
• Organizing types to avoid slow inline complex type expressions

Key Patterns

Pattern 1: Branded Types (Nominal Typing)

Prevent accidentally passing the wrong ID or value:

type Brand<T, TBrand> = T & { __brand: TBrand };
type UserId = Brand<number, "UserId">;
type OrderId = Brand<number, "OrderId">;

function createUserId(id: number): UserId { return id as UserId; }
function getUser(id: UserId) { /* ... */ }

const userId = createUserId(123);
const orderId = 456 as OrderId;

getUser(userId);  // OK
getUser(orderId); // Type error -- caught at compile time

Use cases: Database IDs, monetary values (USD, EUR), validated strings (Email, URL).

Pattern 2: Discriminated Unions with Exhaustiveness Checking

type Shape =
  | { type: "circle"; radius: number }
  | { type: "rectangle"; width: number; height: number }
  | { type: "square"; size: number };

function calculateArea(shape: Shape): number {
  switch (shape.type) {
    case "circle": return Math.PI * shape.radius ** 2;
    case "rectangle": return shape.width * shape.height;
    case "square": return shape.size ** 2;
    default:
      const _exhaustive: never = shape;
      throw new Error(`Unhandled shape: ${_exhaustive}`);
  }
}

TypeScript errors when you add a new variant but forget to handle it — perfect for state machines.

Pattern 3: Template Literal Types

type EventNames = "click" | "focus" | "blur";
type EventHandlers = `on${Capitalize<EventNames>}`;
// Result: "onClick" | "onFocus" | "onBlur"

type HTTPMethod = "GET" | "POST" | "PUT" | "DELETE";
type APIRoute = `/${string}`;
type Endpoint = `${HTTPMethod} ${APIRoute}`;
// Enforces: "GET /users", "POST /users/123", etc.

Pattern 4: Const Assertions (Better Than Enums)

// Use as const instead of enum: no runtime overhead, better tree-shaking
const Status = {
  Active: "ACTIVE",
  Inactive: "INACTIVE",
  Pending: "PENDING"
} as const;

type StatusType = typeof Status[keyof typeof Status];
// "ACTIVE" | "INACTIVE" | "PENDING"

function setStatus(status: StatusType) { /* ... */ }
setStatus(Status.Active);  // OK
setStatus("INVALID");      // Type error

Pattern 5: Custom Type Guards

function isNotNull<T>(value: T | null): value is T {
  return value !== null;
}

const mixed: (string | null)[] = ["a", null, "b", null];
const strings: string[] = mixed.filter(isNotNull);
// Type: string[] -- TypeScript knows nulls are gone

Pattern 6: unknown vs any

// WRONG: 'any' disables type safety -- errors cascade silently
function processInput(data: any) {
  data.nonExistentMethod(); // No error, but crashes at runtime
}

// CORRECT: 'unknown' forces narrowing first
function processInput(data: unknown) {
  if (typeof data === "string") {
    return data.toUpperCase(); // Safe
  }
  throw new Error("Expected string");
}

Pattern 7: Runtime Validation with Zod

// WRONG: type assertion without validation -- silently wrong
const user = JSON.parse(apiResponse) as User;

// CORRECT: validate at the boundary
import { z } from "zod";
const UserSchema = z.object({
  id: z.number(),
  name: z.string(),
  email: z.string().email()
});
const user = UserSchema.parse(JSON.parse(apiResponse));

Pattern 8: Named Types for Performance

// SLOW: TypeScript re-evaluates complex inline types every use
function processData(input:
  Pick<Omit<User, "password">, "id" | "name"> & { role: string }
) { }

// FAST: TypeScript caches named types (30%+ speedup on large codebases)
type UserBasicInfo = Pick<Omit<User, "password">, "id" | "name">;
type UserWithRole = UserBasicInfo & { role: string };
function processData(input: UserWithRole) { }

tsconfig.json Performance Settings

{
  "compilerOptions": {
    "incremental": true,
    "skipLibCheck": true,
    "isolatedModules": true,
    "strictFunctionTypes": true
  }
}

incremental: 30-50% faster rebuilds. skipLibCheck: 20-40% speed boost. isolatedModules: 32% faster compilation.

Migration Strategy: JavaScript to TypeScript

Phase 1 (Day 1): { "allowJs": true, "checkJs": false, "strict": false }

Phase 2 (Weeks 1-4): Convert bottom-up (no-dependency modules first). Use any temporarily.

Phase 3 (Month 2): { "checkJs": true, "strict": true, "noImplicitAny": true }

Utility Types Reference

type User = { id: number; name: string; password: string };
type PartialUser = Partial<User>;          // all fields optional
type ReadonlyUser = Readonly<User>;        // all fields readonly
type PublicUser = Omit<User, "password">; // remove specific fields
type LoginFields = Pick<User, "id" | "name">;
type UserMap = Record<string, User>;

Tools & References

• TypeScript Official Docs
• Total TypeScript -- Matt Pocock
• TypeScript Performance Wiki
• Zod -- Runtime Schema Validation
• npm install -D typescript ts-node @types/node
• npx tsc --init -- generate tsconfig.json

Published by MidOS — MCP Community Library