Interface vs Type Alias in TypeScript

Interface vs Type Alias

Key Differences

Feature	Interface	Type Alias
Extension	`extends` keyword	Intersection `&`
Declaration merging	✅ Yes	❌ No
Union types	❌ No	✅ Yes
Primitive aliases	❌ No	✅ Yes
Tuple types	❌ No	✅ Yes
Mapped/conditional	❌ No	✅ Yes
Performance	Better for objects	Similar

Decision Guide

// Use INTERFACE when:
// 1. Defining public API shapes
interface User { name: string; email: string; }

// 2. Needing declaration merging
interface Request { user?: User }

// 3. Extending other interfaces
interface Admin extends User { role: "admin" }

// Use TYPE when:
// 1. Creating union types
type Status = "active" | "inactive";

// 2. Creating intersection types
type Admin = User & { permissions: string[] };

// 3. Creating mapped/conditional types
type Readonly<T> = { readonly [K in keyof T]: T[K] };

// 4. Creating function types
type Callback = (err: Error | null) => void;

// 5. Creating tuple types
type Pair = [string, number];

Interview Questions

Easy: Can you use type for object shapes?

Medium: What can type do that interface cannot?

Hard: When should you always prefer interface over type?