What does GraphQL do with the return value from a resolver?

Question

I recently started learning GraphQL and I've discovered a situation that I'm unable to explain.

Take the following example

var {ApolloServer, gql} = require("apollo-server")

const typeDefs = gql`
    type Query {
        test: type1!
    }
    type type1 {
        value: String!
    }
`;

const resolvers = {
    Query: {
        test: (parent) => {
            console.log(parent)
            return "How does my query still work?!"
        }

    },
    type1: {
        value: (parent) => {
            console.log(parent)
            return "hello!"
        }

    }
}

const server = new ApolloServer({ typeDefs, resolvers })
server.listen(8080).then(({url}) => console.log(`server stated on ${url}`))

Under normal circumstances, GraphQL resolves the a query from the outside to the inside and will use the most specific resolver it has for a given field until all fields have been resolved.

What I find perplexing about this example is what exactly happens to the string that I return from my top level resolver.

This query

query {
    test {
        value
  }
}

yields the result

{
  "data": {
    "test": {
      "value": "hello!"
    }
  }
}

Given the above example, I would fully expect my GraphQL server to throw an error since my top level resolver returns a String instead of the type1 that the schema asks for.

My question is basically, what is my server doing with the values that my resolvers return? Clearly, if I return a String in my top level resolver, something pretty big must be happening for that to translate into an Object for the query response.

Answer 1

I encountered the same thing today. I didn't understand what's the need for providing a return type if you can return anything like a string for a specific type definition?

So if I see how REST API is done then I can think that the return type is for API documentation and for the application that is consuming it which is hinted in the HTTP content-type header. This tells you an example the return type is of type 'application/json' but not the schema - maybe there is a different header for it? or the application can get it from some kind of API documentation like Swagger.

Useful Swagger doc snippet from https://swagger.io/docs/specification/describing-responses/

paths:
  /users:
    get:
      summary: Get all users
      responses:
        '200':
          description: A list of users
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ArrayOfUsers'
            application/xml:
              schema:
                $ref: '#/components/schemas/ArrayOfUsers'
            text/plain:
              schema:
                type: string

PS: Very new to the web domain. Kindly let me know of any incorrect assumptions.