TypeScript Patterns
Overview
Advanced TypeScript patterns that use the type system to prevent runtime errors. Focuses on strict mode TypeScript with patterns for type inference, narrowing, and compile-time validation. Not a beginner tutorial.
When to use: Building type-safe APIs, complex data transformations, library authoring, preventing runtime errors through types, working with strict mode flags.
When NOT to use: Learning TypeScript basics (primitives, interfaces, classes), JavaScript-to-TypeScript migration guidance, tooling setup, or build configuration.
Quick Reference
| Pattern | API | Key Points |
|---|---|---|
| Utility types | Pick<T, K>, Omit<T, K> | Extract or exclude properties |
| Partial/Required | Partial<T>, Required<T> | Make properties optional or required |
| Record type | Record<K, V> | Object with known keys |
| Awaited type | Awaited<T> | Unwrap Promise return types |
| ReturnType/Parameters | ReturnType<F>, Parameters<F> | Extract function types |
| Generic constraints | <T extends Type> | Constrain generic parameters |
| Type inference | type Inferred = typeof value | Let TypeScript infer from values |
| Type guards | typeof, instanceof, in | Narrow types at runtime |
| Custom type guards | (x): x is Type | User-defined narrowing functions |
| Inferred predicates | (x) => x!== null | Auto-inferred type predicate filters |
| Discriminated unions | Union with literal type property | Exhaustive pattern matching |
| Conditional types | T extends U? X: Y | Type-level conditionals |
| Mapped types | {[K in keyof T]: T[K]} | Transform all properties |
| Template literals | ` ${A}-${B} ` | String literal type manipulation |
| Const assertions | as const | Narrowest possible literal types |
| Satisfies operator | value satisfies Type | Type check without widening |
| NoInfer utility | NoInfer<T> | Prevent inference from a position |
| Const type params | <const T extends Type> | Narrow inference without as const |
| Inline type imports | import {type User} | Import types explicitly |
| Module augmentation | declare module 'lib' {...} | Extend third-party types |
| Assertion functions | function assert(x): asserts x is T | Throw if type guard fails |
Common Mistakes
| Mistake | Correct Pattern |
|---|---|
Using any without justification | Use unknown and narrow with type guards |
| Manual type assertions everywhere | Let inference work, type function returns |
| Destructuring before type narrowing | Keep object intact for discriminated unions |
| Index access without undefined check | Enable noUncheckedIndexedAccess |
as Type casting unsafe values | Use satisfies Type to preserve narrow types |
| Inline objects in generics | Extract to const or type alias |
Omitting extends in constraints | Always constrain generics when possible |
Using Parameters<typeof fn>[0] | Type the parameter directly in function |
| Not handling union exhaustiveness | Use never checks in switch/if-else |
value as const satisfies Type | Use satisfies Type then as const if needed |
Delegation
- Pattern discovery: Use
Exploreagent to find existing patterns in codebase - Type error debugging: Use
Taskagent for multi-step type resolution - Code review: Delegate to
code-reviewerskill for type safety audit
References
- Type utilities (Pick, Omit, Partial, Required, Record, Extract, Exclude, ReturnType, Parameters, Awaited)
- Generics (constraints, inference, default parameters, variance)
- Type guards and narrowing (typeof, instanceof, in, custom guards, assertion functions)
- Discriminated unions (exhaustive checking, never type, tagged unions)
- Conditional types (distributive conditionals, infer keyword, type-level logic)
- Mapped types (key remapping, template literals, modifiers)
- Strict mode patterns (noUncheckedIndexedAccess, exactOptionalPropertyTypes, const assertions, satisfies)
- Module patterns (inline type imports, declaration files, module augmentation, ambient types)
- Modern idioms (eslint-plugin-unicorn patterns, modern array/string/DOM APIs, ES modules)