How to implement isTypeOf method?

isTypeOf is a function that is passed to the constructor of a GraphQLObjectType when you create your schema programatically. Ditto for resolveType functions and unions/interfaces. If you use SDL and create your schema using buildSchema, there is no way to inject those functions into your created schema, just like you don't have a way to provide resolvers for fields on types other than Query and Mutation.

You have a couple of options. One option is to utilize the default resolveType behavior. This checks for a __typename property on the object, and falls back to calling isTypeOf on every implementing type until it matches. That means if you're using a class, it should be sufficient to do something like this:

export default class Todo {  get __typename() {    return 'Todo'  }}

The better option would be to drop buildSchema and use makeExecutableSchema from graphql-tools. Then you can define your resolveType and/or isTypeOf functions directly in your resolvers. For example:

const resolvers = {  Query: {    node: (obj, args, context, info) => {      return new Todo('1', 'Todo 1')    }  },  INode: {    __resolveType: (obj, context, info) => {      if (obj instanceof Todo) return 'Todo'    },  }}

Not only can you easily define isTypeOf or resolveType this way, you can also easily add resolvers for fields of any type and add custom scalars without any hassle. You cannot do any of that (easily) if you're using just buildSchema.

Edit:

If you prefer to utilize isTypeOf instead of resolveType, the resolvers would look something like this:

const resolvers = {  Query: {    node: (obj, args, context, info) => {      return new Todo('1', 'Todo 1')    }  },  Todo: {    __isTypeOf: (obj, context, info) => {      return obj instanceof Todo    },  }}

Only one or the other is necessary. Either write a resolveType function for every abstract type you use, or write a isTypeOf for every object type that could be an abstract type.