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 Explore agent to find existing patterns in codebase
Type error debugging: Use Task agent for multi-step type resolution
Code review: Delegate to code-reviewer skill for type safety audit

typescript-patterns

TypeScript Patterns

Overview

Quick Reference

Common Mistakes

Delegation

References