Example of how you can create a mock server using an existing servers introspection info.

github-graphql-mock-server.js

const express = require('express');
const { graphqlHTTP } = require('express-graphql');
const { getIntrospectionQuery, buildClientSchema } = require('graphql');
const { request } = require('graphql-request');
const fs = require("fs");
const util = require("util");
const mocker = require('easygraphql-mock');

const token = "user your own";

(async () => {
    try {
        const schemaFilePath = __dirname + "/schema.json";
        let result = null;
        if (fs.existsSync(schemaFilePath)) {
            result = JSON.parse((await util.promisify(fs.readFile)(schemaFilePath, { encoding: "utf-8" })).toString());
        } else {
            result = await request({
                url: "https://api.github.com/graphql",
                document: getIntrospectionQuery(),
                variables: {},
                requestHeaders: {
                    Authorization: `bearer ${token}`
                },
            });

            await util.promisify(fs.writeFile)(schemaFilePath, JSON.stringify(result, null, 4), { encoding: "utf-8" })
        }

        const schema = buildClientSchema(result);

        const rootValue = {
            repository: async (args, { schemaCode }) => {
                const mock = mocker(schemaCode, {
                    Repository: {
                        name: args.name
                    },
                    RepositoryOwner: {
                        login: args.owner
                    }
                })

                return mock.Repository;
            }
        };

        const app = express();
        app.use('/graphql', graphqlHTTP({
            schema: schema,
            rootValue: rootValue,
            graphiql: true,
            context: { schemaCode: result }
        }));
        app.listen(5000);
        console.log('Running a GraphQL API server at http://localhost:5000/graphql');
    } catch (e) {
        console.error(e)
    }
})();

query

query {
  repository (owner: "Betsson", name:"a") {
    name
    isPrivate
   	diskUsage
 		hasProjectsEnabled
    owner {
      login
    }
  }
}

Should return:

{
  "data": {
    "repository": {
      "name": "a",
      "isPrivate": false,
      "diskUsage": 4,
      "hasProjectsEnabled": true,
      "owner": {
        "login": "Betsson"
      }
    }
  }
}