24. Generic constraints (extends)

A generic type parameter can be constrained with `extends`, limiting which types are allowed and unlocking property access that would otherwise be unsafe.

ts
interface HasId { id: string; }

function findById<T extends HasId>(items: T[], id: string): T | undefined {
  return items.find(item => item.id === id);
}

interface Token extends HasId { symbol: string; }
const tokens: Token[] = [{ id: "1", symbol: "ETH" }];
findById(tokens, "1"); // T inferred as Token

Because `T extends HasId`, the compiler allows `item.id` inside the function body. Without the constraint, `T` could be anything, and `.id` wouldn't be guaranteed to exist. `keyof` is often combined with constraints, e.g. `function pick<T, K extends keyof T>(obj: T, key: K): T[K]`.

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