What to use for data-only objects in TypeScript: Class or Interface?

Question

I have a bunch of data-only "classes" (in .NET world we call them POCO objects) that does not have any methods or even constructors. Examples are Customer, Product, User entities, you name it...

Originally I started using typescript classes but now I'm thinking that declaring them as interface might be better. From performance standpoint, and not only... It's just that in C# we're used to use interfaces for different thing, and for "POCO" (Plain-old-clr-object, or "data-only" object) we use just a class (sometimes even struct).

What is a proper way to declare them in TypeScript?

Note that I mostly understand (I think) technical differences between class and interface (i.e. that interface is a compile-time construct), but I'm trying to find out which one fits this case semantically.

P.S.: I've seen similar questions (like this) but none of them adress this specific issue clearly and definitely, so please don't close this as 'possible duplicate' or 'opinion-based' (cause it isn't) :)

Answer 1

Interface and it's not even close.

People start writing TypeScript and they suddenly think they have to use classes for some reason. But they don't. Classes are an ES6 feature and they work fine, but if it's just data, it's just data.

A major problem with using classes is that they won't serialize/deserialize like you expect over the wire, so things like instanceof checks won't work.

One rule of thumb is that if there's not internal state associated with some methods, and there's no need for traditional OO polymorphism, don't use a class. This even extends to static classes -- use namespace / module instead.

Answer 2

Use classes with parameter properties:

// Immutable data object
class Person {
  constructor(readonly firstName: String, readonly lastName: String) {}
}

// Mutable data object
class Person {
  constructor(public firstName: String, public lastName: String) {}
}