Non-null means exactly what it sounds like — not null. An empty array is not null — it’s still returning a value. Here is a summary table: declaration accepts: | null | [] | [null] | [{foo: ‘BAR’}] ———————————————————————— [Vote!]! | no | yes | no | yes [Vote]! | no | yes | yes … Read more
Look at the documentation: const { graphqlHTTP } = require(‘express-graphql’); Note that it uses destructuring equivalent to: const graphqlHTTP = require(‘express-graphql’).graphqlHTTP; require(‘express-graphql’) returns an object with a property called graphqlHTTP that is the function you want to call. You’re trying to call the object itself as if it was a function.