Top 20 GraphQL Interview Questions and Answers

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GraphQL is a query language for APIs that was developed by Facebook in 2012 and released publicly in 2015. It was created by Lee Byron and his team to give developers more control over the data they request. Instead of getting too much or too little data from REST APIs, GraphQL lets you ask for exactly what you need. It is now used by companies like GitHub, Shopify, and Twitter. If you are applying for roles like frontend developer, backend engineer, or full-stack developer, these GraphQL interview questions and answers can help you prepare.

Fun Fact: In the GraphQL Report 2024, 61.5% of developers surveyed reported using GraphQL in production, while another 15.5% said they were actively exploring it.

Basic Level GraphQL Interview Questions

Here are some GraphQL interview questions and answers that cover the basic concepts every beginner should know.

1. What is GraphQL and how is it different from REST?

GraphQL is a query language for APIs. It lets clients ask for exactly the data they need. 

REST uses multiple endpoints. GraphQL uses one. In REST, you might over-fetch or under-fetch data. In GraphQL, you get just what you ask for. It is flexible and efficient for modern apps.

2. What is a GraphQL schema and what does it contain?

A GraphQL schema defines the shape of the API. It lists all types, queries, mutations, and subscriptions. It acts like a contract between client and server. The schema helps both sides know what data is available and how it is structured.

3. What are queries in GraphQL? Give an example.

Queries are used to read or fetch data. 

A basic example:

{

  user(id: “1”) {

    name

    email

  }

}

This fetches name and email for the user with ID 1.

4. What are mutations in GraphQL and when do you use them?

Mutations are used to change data. I use them when creating, updating, or deleting something. For example, creating a new user or editing a blog post.

5. What is a resolver in GraphQL?

A resolver is a function. It returns the value for a field in a type. When a query hits the server, resolvers decide what data to send back. Each field can have its own resolver.

6. What are scalar and enum types in GraphQL?

Scalar types are basic data types like String, Int, Float, Boolean, and ID. Enums are custom values that are limited to a fixed set. For example, a Status enum might have values like ACTIVE or INACTIVE.

7. How does GraphQL handle nested queries?

GraphQL supports nested queries naturally. You can ask for related data in one go. For example, fetch a user and their posts in a single query. It is simple and clean.

Also Read - Top 50+ REST API Interview Questions and Answers

Intermediate Level GraphQL Interview Questions 

These GraphQL interview questions and answers will help you strengthen your understanding of key concepts and prepare for mid-level roles.

8. How do you implement pagination in a GraphQL API?

You can use two styles: offset-based or cursor-based. Offset-based uses limit and offset fields. Cursor-based is more reliable for large datasets or real-time data. It uses first, after, last, or before. Cursor pagination helps avoid issues with duplicate or missing results during data changes.

9. What is the purpose of using fragments in GraphQL?

Fragments let you reuse parts of a query. Instead of repeating fields across queries, you group them into a fragment. This keeps code clean and avoids repetition. 

For example, I use fragments to fetch user details like name, email, and avatar in multiple queries.

10. How do you use variables in a GraphQL query?

Variables make queries dynamic and reusable. You define variables outside the query and pass them in when making the request. 

For example:

query GetUser($id: ID!) {

  user(id: $id) {

    name

    email

  }

}

You pass $id as a variable in the request payload.

11. How do you manage error handling in GraphQL?

Errors are returned in the errors array in the response. It shows what went wrong. I also use custom error messages to make debugging easier. 

For example, if a user isn’t found, I throw a UserNotFoundError with a clear message. You can log errors or track them using tools like Sentry.

12. How do you implement authentication in a GraphQL server?

I usually use middleware like Express or Apollo plugins. Tokens are sent in the request headers. On the server, I validate the token before running any resolver. If the token is valid, I attach the user to the context. Then resolvers can check who is making the request.

13. What are input types in GraphQL and how are they useful?

Input types are used to send structured data into mutations or queries. Instead of passing many arguments, you group them in an input type. This is cleaner and helps with validation. 

For example, creating a user might use an input like CreateUserInput.

14. How does caching work in GraphQL?

Caching happens at different levels. On the client, tools like Apollo Client store previous query results. On the server, you can cache responses or resolver results. Dataloader is often used to batch and cache database calls. It reduces repeated work.

Advanced Level GraphQL Interview Questions 

Let’s go through some advanced GraphQL interview questions and answers to help you tackle complex scenarios and senior-level interviews.

15. What is schema stitching and how does it work?

Schema stitching combines multiple GraphQL schemas into one unified schema. It is used when services are split across microservices. Each service defines its own schema, and a gateway merges them. You can also customize how types or fields are resolved across services. Tools like GraphQL Tools help with stitching.

16. How do you create and use custom scalar types?

Custom scalars let you define types beyond the built-in ones. I’ve used them for things like DateTime, URLs, or JSON. You define a name, parsing logic, and validation rules. On the server, you implement serialize, parseValue, and parseLiteral functions. These make sure the scalar behaves correctly.

17. What are some common security challenges in GraphQL?

A few issues I have seen: data overexposure, denial of service through complex queries, and missing access control. You need to add field-level permissions, limit query depth, and throttle requests. Always validate user input, even in a typed schema. Also, don’t expose internal schema hints to unauthenticated users.

18. How do you optimize the performance of a GraphQL API?

I focus on batching and caching first. I use Dataloader to batch database calls inside resolvers. On the client side, Apollo caches responses to reduce traffic. I also add query complexity analysis to stop heavy queries. Avoid N+1 problems and use pagination on large lists.

19. How do you handle deeply nested or large query payloads?

You can limit query depth using validation rules. It helps avoid expensive nested queries. I also set limits on complexity and payload size. It is important to paginate lists and keep resolver logic efficient. Some systems allow max execution timeouts as an extra guard.

20. How do you use GraphQL subscriptions for real-time data?

Subscriptions work over WebSockets. I have used them for things like live chat or notifications. The server uses a subscribe function that pushes updates to clients. Tools like Apollo Server and GraphQL Yoga support it well. You need a pub-sub system behind the scenes, like Redis or in-memory queues.

How to Prepare for GraphQL Interview?

Preparing for a GraphQL interview means knowing the basics and thinking like a real developer. Here are some tips to help you get ready:

• Read the official GraphQL docs and understand how queries, mutations and schemas work
• Practice writing real queries using tools like GraphiQL or Apollo Sandbox
• Build a small project with GraphQL to show real experience
• Learn how to handle errors and write custom resolvers
• Understand query optimization, caching and security basics
• Review common GraphQL interview questions and practice saying your answers out loud
• If asked about real-time data, explain subscriptions clearly
• Know how GraphQL compares to REST in real scenarios

Wrapping Up

So, these are the 20 most asked GraphQL interview questions to help you get job-ready. Practice answering them with real examples and keep building your GraphQL skills. The more hands-on you are, the better your chances.

Looking for jobs in GraphQL and other IT roles? Visit Hirist to find opportunities that match your skills.

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