Express integration in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Express GraphQL server response?

Consider this Express server setup using GraphQL:

const express = require('express');
const { graphqlHTTP } = require('express-graphql');
const { buildSchema } = require('graphql');

const schema = buildSchema(`
  type Query {
    hello: String
  }
`);

const root = {
  hello: () => 'Hello world!'
};

const app = express();
app.use('/graphql', graphqlHTTP({
  schema: schema,
  rootValue: root,
  graphiql: false
}));

app.listen(4000);

If a client sends the query { hello } to /graphql, what is the JSON response?

A {"message":"Hello world!"}

B {"error":"Field 'hello' not found"}

C {"data":{"hello":null}}

D {"data":{"hello":"Hello world!"}}

💡 Hint

Remember that GraphQL responses wrap data inside a data field.

The GraphQL server returns the data requested inside a data object. Since the resolver for hello returns 'Hello world!', the response is {"data":{"hello":"Hello world!"}}.

📝 Syntax

intermediate

Which option correctly sets up GraphQL middleware in Express?

Which of the following code snippets correctly integrates GraphQL middleware into an Express app?

A app.get('/graphql', graphqlHTTP({ schema, rootValue, graphiql: true }));

B app.post('/graphql', graphqlHTTP({ schema: schema, root: rootValue }));

C app.use('/graphql', graphqlHTTP({ schema, rootValue, graphiql: true }));

D app.use('/graphql', graphqlHTTP(schema, rootValue, graphiql: true));

💡 Hint

GraphQL middleware is usually added with app.use and takes an options object.

Option C correctly uses app.use with an options object containing schema, rootValue, and graphiql. Option C uses app.get which is not standard for GraphQL middleware. Option C passes arguments incorrectly. Option C uses root instead of rootValue and only listens to POST requests.

❓ optimization

advanced

How to optimize GraphQL schema building in Express for performance?

You notice your Express server rebuilds the GraphQL schema on every request, causing slow responses. Which approach optimizes this?

A Build the schema once outside the request handler and reuse it for all requests.

B Build the schema inside the middleware function on every request.

C Use a new schema instance for each query to avoid caching issues.

D Disable schema caching to ensure fresh data every time.

💡 Hint

Think about expensive operations and how often they should run.

Building the schema is expensive. Doing it once and reusing it improves performance. Building it on every request (option A) slows down the server. Options C and D cause unnecessary overhead.

🔧 Debug

advanced

Why does this Express GraphQL server return a 404 error?

Given this Express setup:

const express = require('express');
const { graphqlHTTP } = require('express-graphql');
const { buildSchema } = require('graphql');

const schema = buildSchema(`
  type Query {
    greet: String
  }
`);

const root = {
  greet: () => 'Hi!'
};

const app = express();

app.get('/graphql', graphqlHTTP({ schema, rootValue: root, graphiql: true }));

app.listen(3000);

When a client sends a POST request to /graphql, the server responds with 404 Not Found. Why?

A The middleware is only attached to GET requests, so POST requests are not handled.

B The schema is invalid causing the server to reject POST requests.

C The root resolver is missing a POST handler.

D The server is not listening on port 3000.

💡 Hint

Check which HTTP methods the middleware is attached to.

The middleware is attached only to GET requests via app.get. POST requests to /graphql are not handled, resulting in 404 errors. Using app.use would handle all HTTP methods.

🧠 Conceptual

expert

What is the role of the rootValue in Express GraphQL integration?

In Express GraphQL middleware, what does the rootValue option represent?

A It defines the GraphQL schema structure and types.

B It provides resolver functions for the fields in the GraphQL schema.

C It configures the HTTP server settings for Express.

D It enables the GraphiQL interface for testing queries.

💡 Hint

Think about what resolves the data for queries.

The rootValue is an object that contains resolver functions. These functions provide the data for the fields defined in the GraphQL schema when queries are executed.