TypeScript Interfaces vs Types

TypeScript Interfaces vs Types

Understanding the differences between interfaces and types in TypeScript and when to use each.

Interfaces

interface User {
  name: string;
  age: number;
  email?: string;
}

Type Aliases

type User = {
  name: string;
  age: number;
  email?: string;
}

Key Differences

1. Declaration Merging

Interfaces support declaration merging:

interface User {
  name: string;
}

interface User {
  age: number;
}

// Results in: { name: string; age: number; }

2. Extending

Both can extend, but syntax differs:

// Interface
interface Admin extends User {
  role: string;
}

// Type
type Admin = User & {
  role: string;
}

When to Use What?

Use Interfaces when:

• Defining object shapes
• Need declaration merging
• Building public APIs

Use Types when:

• Need unions or intersections
• Working with primitives
• Creating utility types