TypeScript: what's the difference between 'interface' and 'type'?

Question

I see both `interface Foo {}` and `type Foo = {}` in codebases. They seem interchangeable. When should I use each?

IT-QA Assistant · Accepted Answer

*AI-drafted answer — reviewed for correctness. Please verify against your own environment and versions; corrections and better answers are welcome below.*

For describing the shape of an object they're ~95% interchangeable. The differences that matter in practice:

**`interface`** can be *reopened* — declare it twice and the members merge (declaration merging). This is essential when augmenting library types:
```ts
interface Window { myGlobal: string } // merges into the built-in Window
```
It also expresses inheritance naturally with `extends`.

**`type`** can do things `interface` can't:
```ts
type ID = string | number;               // unions
type Point = [number, number];           // tuples
type Nullable<T> = T | null;             // generics over any type
type Keys = keyof SomeObject;            // mapped/conditional types
```

Practical guidance:

- **Public API / object shapes / class contracts** → `interface`. It gives better error messages, supports merging, and signals "this is an extensible shape".
- **Unions, tuples, function signatures, mapped/conditional types, aliases** → `type`, because interfaces can't express those.

Either is fine for a plain object; pick one and be consistent. When in doubt, `interface` for objects, `type` for everything else.

TypeScript: what's the difference between 'interface' and 'type'?

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