type vs interface in TypeScript

In TypeScript, both type and interface can be used to describe the shape of objects. However, they have important differences that affect scalability, composability, and predictability—especially in modern Next.js projects using TypeScript.

This guide outlines the key differences and when to prefer type over interface.

✅ Why Use type

1. Supports Unions and Intersections

type makes it easy to compose multiple types together:

type Result = Success | Failure;
type Combined = Base & Extra;

Interfaces cannot express union types.

2. Handles Non-Object Types

type works with primitives, tuples, and function signatures:

type Status = 'idle' | 'loading' | 'error';
type Point = [number, number];
type ClickHandler = (event: MouseEvent) => void;

interface is limited to describing object-like shapes.

3. Immutable by Design

type aliases are closed — you can’t reopen or merge them later:

type User = { id: string };
// Cannot be redefined elsewhere

This helps prevent accidental type merging across files/modules.

🤔 When to Use interface

There are still cases where interface is useful:

• Extending framework types (e.g. React.ComponentProps)
• Public APIs where declaration merging is intentional
• Class-based or OOP-style codebases

✨ Recommendation

For most modern Next.js + TypeScript projects:

✅ Use type by default
🤝 Use interface only when you need structural extendability or declaration merging

Example

interface (legacy style):

interface User {
  id: string;
  name: string;
}

type (recommended):

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

Summary Table

Conclusion

Favoring type ensures better composability, more predictable behavior, and consistency across different kinds of type definitions in a modern TypeScript codebase.