Generics Fundamentals

Generics let you write components that work over many types without losing type safety:

function identity<T>(value: T): T {
  return value;
}

const num = identity(42);       // T is number
const str = identity("hello");  // T is string

A type parameter like T is a placeholder for a concrete type that callers supply or TypeScript infers.

Key generic patterns:

Constraints – Require certain properties:

function getLength<T extends { length: number }>(value: T) {
  return value.length;  // Safe! We know T has length
}

Related type parameters – Link keys to objects:

function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];  // Return type matches the property type
}

Generic types and classes:

type Box<T> = { value: T };
class Stack<T> { /* ... */ }

In this category, you'll:

Note: Type arguments are erased at runtime, so implementations rely on normal JavaScript while types keep the API safe.