addTypenames copies a given input object and recursively adds GraphQL __typenames to the output.

yarn install add-typenames graphql

const typedItem = addTypenames({ item, schema });

import addTypenames from 'add-typenames';
import { buildSchema } from 'graphql'

const typeDefs = `
    """ Add your schema definition here
`;
const item = {
    /* Your item to be typed */
};

const schema = buildSchema(typeDefs);

const typedItem = addTypenames({ item, schema });

Hint: Adding __typename to the root of the input item will resolve most issues that may arise by leveraging the schema to resolve all types rather than relying on heuristics

In order to properly resolve Unions, Interfaces and Scalars we need the resolvers for those types to be included as part of the schema. This can be done with makeExecutableSchema

import { makeExecutableSchema } from '@graphql-tools/schema';

const schema = makeExecutableSchema({
    typeDefs,
    resolvers,
});

Hint: Only include the resolvers for Unions, Interfaces and Scalars. The rest are unused and not needed.