For express it is described in

how to properly close node-express server?

but in documentation of yoga

https://github.com/prisma/graphql-yoga

there is not info about close method.

---

UPDATE

Solution of @perdox solves my problem, correct answer is:

const gqlServer = new GraphQLServer({ typeDefs, resolvers });
const server = gqlServer.start(() => console.log('Server is running on localhost:4000'));
(async function f() {
    (await server).close();
})();

GraphQLYoga is a collection of tools and uses / wraps the express server.

See: https://github.com/prisma/graphql-yoga/blob/master/src/index.ts

So as you can see in the source code the GraphQLServer wraps the express server and the start method returns a HttpServer Promise. So you should be able to close the server by invoking the close method on the HttpServer:

const gqlServer = new GraphQLServer({ typeDefs, resolvers })
const server = gqlServer.start(() => console.log('Server is running on localhost:4000'))
server.close()