Interfaces vs Types in TypeScript

TypeScript provides two primary tools for describing the shape of data: interfaces and type aliases. They often overlap, but each has strengths that make it better suited for different use cases.

This guide explains their differences, where they intersect, and how to choose between them in real production code.

1. What Interfaces Are Best At

Interfaces describe the structure of objects and are ideal when modeling entities, props, or domain objects.

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

Key strengths of interfaces

• They can extend other interfaces
• They merge automatically when declared multiple times
• They are the most semantic choice for object modeling

Example: extending interfaces

interface User {
  id: number;
}

interface Admin extends User {
  role: "admin";
}

Example: interface merging

interface Settings {
  theme: string;
}

interface Settings {
  language: string;
}

const s: Settings = {
  theme: "dark",
  language: "en"
};

Merging is useful for augmenting third‑party types.

2. What Type Aliases Are Best At

Type aliases describe any type, not just objects.

type User = {
  id: number;
  name: string;
};

Types can represent:

• primitives
• functions
• arrays
• tuples
• unions
• intersections
• template literal types

Example: union type

type Status = "loading" | "error" | "success";

Example: function type

type Fetcher = (url: string) => Promise<string>;

Example: tuple

type Point = [number, number];

3. Where Interfaces and Types Overlap

For objects, interfaces and types behave almost the same:

interface User {
  id: number;
}

type User = {
  id: number;
};

Both support:

• readonly fields
• optional fields
• nested structures
• index signatures

If you only need a simple object shape, both options work.

4. Key Differences You Must Know

1. Interfaces can merge. Types cannot.

interface Box {
  size: number;
}

interface Box {
  weight: number;
}

Types with the same name cause an error.

2. Types support unions, intersections, and primitives

Interfaces cannot model:

• unions
• primitives
• template literal types

This makes type aliases more flexible in advanced scenarios.

3. Interfaces are better for extension-heavy systems

Extending is more ergonomic with interfaces:

interface A {
  x: number;
}

interface B extends A {
  y: number;
}

Types can do similar work with intersections:

type A = { x: number };
type B = A & { y: number };

But interfaces remain more readable for object inheritance.

5. When to Use Interfaces vs Types

Use interfaces when:

• modeling structured data
• defining React props
• defining class shapes
• you want declaration merging
• you want simple, readable API definitions

Use type aliases when:

• working with unions
• defining function signatures
• modeling tuples
• combining multiple types
• creating utility or computed types

6. Practical Recommendations for Real Codebases

• Use interfaces by default for objects
• Use types for unions, functions, and compositions
• Do not overthink the distinction
• Keep your codebase consistent by adopting a simple rule

A good rule used in many teams:

> Use interfaces for data. Use types for everything else.

Final Thoughts

Interfaces and type aliases overlap, but they are not interchangeable. Understanding their strengths helps you create cleaner, more maintainable type systems. Use interfaces for object modeling and use type aliases when you need flexibility, unions, or advanced type transformations.