merge.js

// const { makeExecutableSchema } = require('@graphql-tools/schema')
const { stitchSchemas } = require('@graphql-tools/stitch')
const { execute, parse, printSchema, print, buildSchema } = require('graphql')

const aSchema = buildSchema(/* GraphQL */ `
  type Thing {
    id: ID!
    foo: String
  }
  type Query {
    thing(id: ID!): Thing
  }
`)

const bSchema = buildSchema(/* GraphQL */ `
  type Thing {
    id: ID!
    bar: String
  }
  type Query {
    thing(id: ID!): Thing
  }
`)

const schema = stitchSchemas({
  subschemas: [
    {
      schema: aSchema,
      executor: () => {
        console.log('inside a')
        return { data: { thing: { id: 'a', foo: 'faz' } } }
      },
    },
    {
      schema: bSchema,
      executor: () => {
        console.log('inside b')
        return { data: { thing: { id: 'b', bar: 'baz' } } }
      },
    },
  ],
})

console.log(printSchema(schema))

console.log(
  execute({
    schema,
    document: parse('query { thing(id:"XYZ") { id, foo, bar } }'),
  })
)