Interface vs Type Alias

When to use interface and when to use type. Learn extends, declaration merging, and how structural typing decides assignability.

interfacetypeextendsstructural

Duration

⏱ About 1.5 hours

Level

📊 Intermediate

Prerequisite

🎯 ts-02

OUTCOME

Choose interface vs type consistently across a codebase

What you'll learn

1Define object shapes with interface
2Extend interfaces and intersect types
3Explain structural ("duck") typing
4Pick interface for libraries, type for everything else

1. interface

typescript

interface User {
  id: number;
  name: string;
  email?: string;
  greet(): string;
}

const alice: User = {
  id: 1,
  name: "Alice",
  greet() { return `Hi, ${this.name}`; },
};

2. Extending

typescript

interface Animal { name: string }
interface Dog extends Animal { bark(): void }

// Type alias intersection — equivalent
type DogT = Animal & { bark(): void };

3. interface vs type — When

Need	Pick
Object shape, especially for public API	interface
Union, tuple, primitive alias	type
Need declaration merging	interface
Computed / mapped types	type

💡

Many teams pick one consistently and stick with it. Either works for most cases.

4. Structural Typing

typescript

interface Point { x: number; y: number }

function print(p: Point) { console.log(p.x, p.y); }
print({ x: 1, y: 2, color: "red" });   // OK — extra props allowed at call site

// TS asks "does it have the right shape?" — not "is it nominally a Point?"

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗