Understanding TypeScript Generics: From Zero to Production

What Are Generics?

Generics allow you to write reusable components that work with a variety of types rather than a single one. Think of them as type-level functions.

Basic Generic Functions

function identity<T>(arg: T): T {
  return arg
}

The type variable T captures whatever type the user provides, making it available later in the function signature.

Generic Constraints

Sometimes you want to constrain what types can be passed. Use the extends keyword:

function getLength<T extends { length: number }>(arg: T): number {
  return arg.length
}

Real-World Patterns

Generics shine in API response wrappers, repository patterns, and utility types. Here's a typed API response wrapper:

interface ApiResponse<T> {
  success: boolean
  data: T
  message?: string
}

Conditional Types

TypeScript's conditional types let you implement type-level logic: T extends U ? X : Y. Combined with infer, they unlock powerful meta-programming patterns used throughout popular libraries.

Mapped Types

Mapped types transform existing types into new ones. Partial<T>, Required<T>, and Readonly<T> are all implemented as mapped types in the TypeScript standard library.