Discussion on: TypeScript: type vs interface

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One more thing: as I said interface allows extension even after declaration, so it would be possible extend interface exposed by 3-rd party library, but I can't imagine good use case for it. Any ideas?

Vítor Castro • Jul 22 '19

There’s an example of the usefulness of this in a older than the current version of a functional programming library fp-ts.
It’s called declaration merging and it was used in that library, that implements higher kinded types, to extend those types.

stereobooster • Jul 22 '19

Can you please point me to the code (link to the file/commit in github)? I want to understand it better

Vítor Castro • Jul 24 '19 • Edited

Yes, you can have a look here:
github.com/gcanti/fp-ts/tree/1.x

Has I explained this particular feature was used extensively in V1, but I think that is no longer the case due to the way the library is now structured.

David Pereira • Nov 20 '19

An example where you'd want to extend the interface of a 3rd party API is OrderCloud. They have the property xp to enable you to extend their data model. Here's an example of a type where xp is object (I think sometimes it's any too).
And the extension would be something like:

interface OrderExtended extends Order {
    xp: OrderXp | null
}
export interface OrderXp {
  isCool: boolean
  isAwesome: boolean
  maybeIsBoth: boolean
}