If you’re using TypeScript, you’ve likely come across both type and interface. While they can often be used interchangeably to define the shape of data, there are subtle differences that can affect how and when you use them.
What They Have in Common
Both type and interface let you describe the structure of objects, functions, or other data shapes. In most day-to-day use cases, either one will get the job done.
Key Differences
| Feature | type | interface |
|---|---|---|
| Extension | Can combine multiple types using union and intersection (&) | Can combine multiple types using union and intersection (&) |
| Merging | Cannot be re-declared — it’s fixed after definition | Interfaces with the same name are automatically merged |
| Use Case | Great for union types, primitives, tuples, etc. | Ideal for defining object shapes and class contracts |
| Flexibility | More versatile for complex types | Better for structural and OO-style designs |
Which Should You Use?
-
Use
interfacewhen working with objects or class structures. -
Use
typewhen you need unions, intersections, or more flexible combinations.
In practice, consistency is more important than perfection. Pick one that fits your style or team conventions — and stick with it.