TypeScript Generics: Interview Questions and Practical Examples

Generics provide the foundation for reusable type-safe libraries in TypeScript. Interview questions often evaluate whether candidates can write parameterized functions, enforce constraints using extends, and infer return types dynamically.

What Are Generics?

Generics enable functions, interfaces, and classes to accept type parameters:

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

const num = identity(42); // Inferred as number const str = identity("hello"); // Inferred as string ```

Generic Constraints (extends)

Restrict accepted type arguments using the extends keyword:

interface HasLength {
  length: number;

function logLength(item: T): number { console.log(item.length); return item.length; }

logLength("hello"); // OK (strings have .length) logLength([1, 2, 3]); // OK (arrays have .length) // logLength(123); // Error: Argument of type 'number' is not assignable to 'HasLength' ```

Combining Generics with keyof

function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];

const user = { id: 1, name: "Alice", active: true }; const name = getProperty(user, "name"); // Inferred as string // const invalid = getProperty(user, "foo"); // Error: Argument of type '"foo"' is not assignable to keyof user ```

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