import { importSchema } from 'graphql-import'
import { makeExecutableSchema } from 'graphql-tools'

const typeDefs = importSchema('schema.graphql'); // or .gql or glob pattern like **/*.graphql
const resolvers = {};

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

Assume the following directory structure:

.
├── schema.graphql
├── posts.graphql
└── comments.graphql

schema.graphql

# import Post from "posts.graphql"

type Query {
  posts: [Post]
}

posts.graphql

# import Comment from 'comments.graphql'

type Post {
  comments: [Comment]
  id: ID!
  text: String!
  tags: [String]
}

comments.graphql

type Comment {
  id: ID!
  text: String!
}

Running importSchema('schema.graphql') produces the following output:

type Query {
  posts: [Post]
}

type Post {
  comments: [Comment]
  id: ID!
  text: String!
  tags: [String]
}

type Comment {
  id: ID!
  text: String!
}