How can the resolveType function of unionType or interfaces isTypeOf be made more type-safe?

sikanhe / gqtx

Code-first Typescript GraphQL Server without codegen or metaprogramming

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For interfaces, the best thing to do is using isTypeOf.

E.g.

const isTypeOf = (src: any) => src?.type === "Operator"

With a library such as io-ts the stuff can be even "more" save:

import { flow } from "fp-ts/lib/function";
import * as t from "io-ts";
import * as E from "fp-ts/lib/Either";

const NoteModel = t.type({
  id: t.string,
  title: t.string,
  content: t.string,
  createdAt: t.number,
  updatedAt: t.number,
});

const isTypeOf = flow(
  NoteModel.decode,
  E.fold(
    () => false,
    () => true
  )
);

The drawback of this is, that the type-safety does happen while executing the code not while compiling it. I wonder whether there are other solutions worth exploring.

For a union type we have the same "issue":

const resolveType = (input) => {
  if (isTypeOfNote(input)) { 
    return GraphQLNoteType;
  } else if (isTypeOfImage(input)) {
    return GraphQLImageType;
  }
  throw new Error("Could not resolve type.");
}

I mentioned some more stuff related to this in https://github.com/sikanhe/gqtx/issues/9

sikanhe / gqtx

How can the resolveType function of unionType or interfaces isTypeOf be made more type-safe? #21