If you are a frontend developer and you are using TypeScript, you have probably asked yourself: "Should I use type or interface?"

At first they look exactly same. They both help you define how you want an object to look so your code doesn't crash. But as your React or Next.js project gets bigger, those small differences start to matter, performance and clean coding.
I will explain these differences in this article so you can choose which one to use in less than 5 minutes

1. The Quick Comparison

At their core, both tools act as a "contract" for your data. Here is how they look side-by-side:

2. Why "Interface" is Great for Performance

The biggest internal difference is how the TypeScript compiler (the program that checks your code for errors) addresses them.
TypeScript is clever when you use a Interface with the extends keyword. It caches (saves) that interface, by name, into an internal registry. This makes it very fast to check your code later as the compiler doesn't have to 're-read' the whole structure each time.
On the other hand, ** Types ** often use the " intersection " operator (&). Often the compiler has to recompute the whole shape every time it sees that type, instead of using a fast cache.
You won't see this in a small project. But in a huge enterprise app with thousands of types, interfaces can make your build times 2x faster.

Interfaces have a unique feature called Declaration Merging. This means if you define the same interface twice, TypeScript simply merges them into one.
TypeScript

interface Window {
  myCustomTheme: string;
}

interface Window {
  isLoggedIn: boolean;
}

// Result: Window now has both properties!
This is the "glue" of the TypeScript ecosystem. It allows you to add new properties to third-party libraries or global objects (like window or process.env) without changing their original code.
Types cannot do this. If you try to declare the same type name twice, TypeScript will give you a "Duplicate identifier" error. Thus npm packages mostly use interface.

3. Why "Type" is More Flexible

While interfaces are faster and mergeable, type is the clear winner for complex logic.

A type can be anything: a string, a number, a union (this OR that), or a tuple (a fixed-length array). Interfaces are strictly for objects.

• Union Types: Essential for modern state management.

  Example: a button that can only be 'primary', 'secondary', or 'danger'.

• Utility Types: Enable advanced features like Partial<T> or Omit<T>, which save a lot of time when writing React components.

Best Practices: When to use which?

Use type (most of the time):

• Works with unions, intersections, primitives, tuples
• More flexible for modern React patterns
• Cleaner when composing types

type ButtonProps = {
  variant: "primary" | "secondary";
  onClick: () => void;
};

// You must use type for 
type Status = "idle" | "loading" | "error"; // union → interface can't do this

Use interface (specific cases):

• When you want extending / merging
• Better for object shapes that may grow over time
• Useful in library/public API design

interface User {
  id: string;
}

interface Admin extends User {
  role: string;
}

// or for custom declaration merging
interface Window {
  myCustomProp: string;
}

In React specifically

• Props → either works, but most teams now prefer type
• Complex props (unions, conditional types) → type wins
• Simple object shapes → either, doesn’t matter

Looking Forward: TypeScript 7.0

A major update is coming with TypeScript 7.0. Microsoft is rewriting the compiler in Go, expected to make it 10x faster.

This could eliminate the performance gap between type and interface. When build times drop significantly, you can simply choose whichever improves readability.

Final Summary

Don’t overthink it.

Most modern teams default to type because it’s more flexible and safer. Use interface only when you specifically need extendability or structured inheritance.

The key: stay consistent across your team.