How to use the apollo-server.mergeSchemas function in apollo-server

export default function createApolloServer(options = {}) {
  const { context: contextFromOptions, expressMiddleware, resolvers } = options;
  const path = options.path || DEFAULT_GRAPHQL_PATH;

  // We support passing in either a typeDefs string or an already executable schema,
  // for the case where a plugin is stitching in a schema from an external API.
  const schemas = options.schemas || [];
  const schemasToMerge = schemas.filter((td) => typeof td !== "string");
  const typeDefs = schemas.filter((td) => typeof td === "string");

  // Create a custom Express server so that we can add our own middleware and HTTP routes
  const app = express();

  let schema = makeExecutableSchema({ typeDefs, resolvers, resolverValidationOptions });
  if (schemasToMerge.length) {
    schema = mergeSchemas({ schemas: [schema, ...schemasToMerge] });
  }

  const apolloServer = new ApolloServer({
    async context({ connection, req }) {
      const context = { ...contextFromOptions };

      // For a GraphQL subscription WebSocket request, there is no `req`
      if (connection) return context;

      // Express middleware should have already set req.user if there is one
      await buildContext(context, req);

      await createDataLoaders(context);

      return context;
    },

// We support passing in either a typeDefs string or an already executable schema,
  // for the case where a plugin is stitching in a schema from an external API.
  const schemas = options.schemas || [];
  const schemasToMerge = schemas.filter((td) => typeof td !== "string");
  const typeDefs = schemas.filter((td) => typeof td === "string");

  if (typeDefs.length === 0 && schemasToMerge.length === 0) {
    throw new Error("No type definitions (schemas) provided for GraphQL");
  }

  // Create a custom Express server so that we can add our own middleware and HTTP routes
  const app = express();

  let schema = makeExecutableSchema({ typeDefs, resolvers, resolverValidationOptions });
  if (schemasToMerge.length) {
    schema = mergeSchemas({ schemas: [schema, ...schemasToMerge] });
  }

  const apolloServer = new ApolloServer({
    async context({ connection, req }) {
      const context = { ...contextFromOptions };

      // For a GraphQL subscription WebSocket request, there is no `req`
      if (connection) return context;

      // Express middleware should have already set req.user if there is one
      await buildContext(context, req);

      await createDataLoaders(context);

      return context;
    },

export async function createAndMergeRemoteSchemas() {
    const schemas = await Promise.all(["cms/drupal/web/graphql"].map(createRemoteSchema));
    const filteredSchemas = schemas.filter((schema) => schema !== undefined);
    if (filteredSchemas.length) {
        return mergeSchemas({schemas: filteredSchemas});
    } else {
        throw new NoRemoteSchemasFound("Couldn't generate remote schemas from any service.");
    }
}