TypeScript Advanced Types

Modern TypeScript playbook for writing code that is correct, ergonomic, and fast enough.

Quick Reference

Need	See
Modern features (satisfies, const type params)	Modern Features
Generics, conditional types, mapped types	Type-Level Toolbox
Config objects, event emitters, API clients	Practical Patterns
Test patterns, tsconfig templates	Tests & Configs
Compile-time optimization	Performance Playbook
Type testing, PR checklist	Type Testing

When to Use

Use this skill when:

• Building type-safe configs/maps/APIs (feature flags, route tables, event maps)
• Designing type-safe APIs (HTTP clients, RPC layers, CLI command maps)
• Enforcing strictness beyond "strict": true:
  • noUncheckedIndexedAccess
  • exactOptionalPropertyTypes
  • verbatimModuleSyntax
• Improving compile times (incremental builds, project references)
• Setting up separate TS/ESLint configs for tests

Default Workflow

1. Start with data shape and ownership — Is value internally constructed (safe) or external input (unsafe)?
2. Choose the lightest typing tool — satisfies for configs, mapped types for transformations, infer for function inference
3. Lock strictness and hygiene — Enable noUncheckedIndexedAccess, exactOptionalPropertyTypes, consider verbatimModuleSyntax
4. Keep builds fast — Enable incremental, use project references in monorepos, measure with --extendedDiagnostics
5. Tests: strict where matters, pragmatic where not — Separate tsconfig.test.json, ESLint overrides for test files only

Key Highlights

satisfies — Use Aggressively

Validates object shape without losing literal inference or widening to string/number.

type AppConfig = {
  env: "dev" | "prod";
  apiBaseUrl: string;
  retries: number;
};

export const config = {
  env: "dev",
  apiBaseUrl: "https://api.example.com",
  retries: 3,
} satisfies AppConfig;
// config.env is "dev" (literal), not "dev" | "prod"

Strictness Beyond "strict": true

noUncheckedIndexedAccess — Adds undefined to indexed access:

const dict: Record<string, { id: string }> = {};
const x = dict["missing"]; // { id: string } | undefined

exactOptionalPropertyTypes — Optional ≠ undefined:

type UserPrefs = { theme?: "dark" | "light" };
const prefs: UserPrefs = {};
prefs.theme = "dark";     // ok
// prefs.theme = undefined; // error

verbatimModuleSyntax — Module hygiene:

import type { User } from "./types";     // type-only
import { createUser } from "./runtime"; // runtime (kept in output)

Resources

See references/ for detailed guides on each topic.