21 Top GraphQL Interview Questions and Answers (2024)

Blog / 21 Top GraphQL Interview Questions and Answers (2024)

GraphQL is a modern query language that's revolutionizing how developers interact with APIs.

Not only is GraphQL used widely inside many of the worlds larges tech companies, its power and flexibility has made it a must have for many up and coming startups as well!

Studying GraphQL is a no brainer if you want to ensure that you stand out when applying for backend and full stack developer positions.

Q1.

What is GraphQL?

Junior

GraphQL is an open-source query language and runtime for APIs. GraphQL allows you to define a schema that describes your data, and clients can then use that schema to request exactly the data they need, in a single round-trip to the server.
It provides a more efficient, powerful, and flexible alternative to the traditional REST API.

Q2.

Explain the basic components of a GraphQL schema.

Junior

A GraphQL schema consists of several key components:
- Types: Define the objects and their fields in your data. Example: User type with fields id, name, email.
- Fields: Attributes of types that specify the data you can query.
- Scalar Types: Basic data types like Int, String, Boolean, without sub-fields.
- Query Type: Defines read operations and entry points for querying the data.
- Mutation Type: Specifies write operations for modifying data.
- Subscription Type: Allows real-time updates through specified events.
- Enums: Special scalar type restricted to a predefined set of values.
- Interfaces and Unions: Define fields that multiple types must include (interfaces) or allow for more dynamic schemas (unions).
- Input Types: Specify complex objects as inputs for queries and mutations.
- Directives: Modify the execution of queries and mutations, like @include or @skip.
The schema, defined using the GraphQL Schema Definition Language (SDL), acts as a contract between the server and client, outlining how data can be accessed and manipulated.

Q3.

What is a GraphQL query?

Junior

A GraphQL query is a request to read specific data from a GraphQL server. It allows clients to precisely specify what data they need. Key features include:
- Selective Data Fetching: Clients can request exactly what they need, avoiding over-fetching or under-fetching.
- Hierarchical and Strongly Typed: Queries reflect the structure and relationships in the data model, and are validated against the GraphQL schema for correctness.
- Field-Level Requests: Clients can query specific fields on objects.
- Nested Objects: Ability to fetch complex data structures in a single request.
- Single Endpoint: All data requests are sent through a single API endpoint.
Example:

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Q4.

What is a GraphQL mutation?

Junior

A GraphQL mutation is used for modifying data on the server, such as creating, updating, or deleting records. Key aspects include:
- Write Operations: Specifically for INSERT, UPDATE, or DELETE actions.
- Return Values: Can return data to provide the new state of an object after the mutation.
- Sequential Execution: Executed in order, ensuring data consistency.
- Schema-Defined: Mutations are defined in the GraphQL schema with specified inputs and return types.
- Strongly Typed: Inputs and outputs are strictly validated against the schema.
Example:

Q5.

Explain GraphQL types.

Junior

GraphQL has a type system that defines the structure and relationships of the data in the API. Types can be scalar types (like strings or integers) or object types (which represent more complex data structures).
Example:

Q6.

What is a GraphQL resolver?

Mid

A GraphQL resolver is a function that resolves the value for a specific field in a schema. It acts as a bridge between the GraphQL server and the data source, executing the logic necessary to return the requested data. Resolvers are defined for each field and can handle data retrieval, computation, or interacting with various backends.
Example:

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Q7.

Explain the main differences between REST and GraphQL?

Mid

Data retrieval: REST APIs typically expose a fixed set of endpoints, each of which returns a fixed set of data. In contrast, GraphQL APIs allow clients to specify exactly what data they need, using a single flexible endpoint.
Data manipulation: In a REST API, data is typically manipulated using HTTP verbs such as POST, PUT, PATCH, and DELETE, with each verb corresponding to a specific type of operation. In a GraphQL API, data is manipulated using mutations, which can be thought of as a generalization of these HTTP verbs.
Data structure: REST APIs are resource-oriented, which means that they organize data into resources that can be accessed using URLs. In contrast, GraphQL APIs organize data into a graph-like structure, which allows for more complex relationships between data.
Client-server relationship: In a REST API, the client has a passive role and relies on the server to provide the necessary resources. In contrast, in a GraphQL API, the client has an active role and can request exactly the data it needs, which can reduce overfetching and underfetching of data.

Q8.

Explain middleware in the context of GraphQL.

Mid

In the context of GraphQL, middleware refers to a layer of software that sits between the GraphQL server and the resolver functions. It provides a way to modify, enhance, or manage the processing of requests and responses in the GraphQL execution flow. Key aspects of middleware in GraphQL include:
- Request Processing: Middleware can intercept and process requests before they reach the resolver functions.
- Response Transformation: Middleware can also modify or augment the response from resolver functions before it's sent back to the client.
- Resolver Wrapping: Middleware often wraps around resolver functions, allowing you to execute code before and/or after the resolver logic.
- Modular Architecture: By encapsulating specific functionalities in middleware, it promotes a modular and maintainable codebase.

Q9.

How do you implement authentication and authorization in GraphQL?

Mid

Authentication:
- Typically handled outside GraphQL, often using HTTP Authorization headers with tokens (like JWT).
- The server validates the token and attaches user information to the context accessible in resolvers.
- Example:
Authorization:
- Implemented in resolvers, where you check the user's roles or permissions (from the context) before processing the request.
- Can also use GraphQL directives for declarative authorization rules in the schema, or Role-Based Access Control (RBAC) for finer control.
- Example:

Q10.

What are GraphQL fragments?

Mid

GraphQL fragments are reusable pieces of query logic that can be shared between multiple queries or within a single query to avoid repetition. They allow you to define a set of fields that you want to retrieve from one or more types, and then include this set wherever needed. Fragments make your queries more maintainable and readable, especially when dealing with complex data structures.
Example:
- In this example, the userInfo fragment is defined to include the id, name, and email fields of a User. It is then reused in the getUser query.

Q11.

How are errors handled in GraphQL?

Mid

Errors in GraphQL can be handled by returning an error object from the resolver for a particular field.
A successful GraphQL query is supposed to return a JSON object with a root field called "data". If the request fails or partially fails (e.g. because the user requesting the data doesn’t have the right access permissions), a second root field called "errors" is added to the response:

Q12.

What is the difference between Relay and Apollo in the context of GraphQL?

Mid

Relay and Apollo are both GraphQL client libraries, but with different focuses:

Relay:
- Optimized for performance, especially in complex React applications.
- Uses a compiler for GraphQL queries, emphasizing build-time optimizations.
- Imposes strict conventions on GraphQL queries and schema design.
- Best suited for large-scale applications with sophisticated data requirements.
Apollo:
- Known for flexibility and ease of use across various JavaScript frameworks.
- Offers a comprehensive feature set including caching, state management, and subscriptions.
- Allows incremental adoption, making it easier to integrate into existing projects.
- Has a large community and is more accessible, suitable for a wide range of applications.
Key Differences:
- Relay is more performance-focused and convention-driven, ideal for complex React projects, while Apollo offers greater flexibility and a broader feature set, catering to diverse needs and frameworks.

Q13.

What is a GraphQL subscription?

Mid

A GraphQL subscription is a feature in GraphQL that enables the client to receive real-time updates from the server. Unlike queries and mutations that follow a request-response cycle, subscriptions maintain a persistent connection to the server (commonly using WebSocket protocol). This allows the server to push updates to the client as soon as events happen. Subscriptions are particularly useful in scenarios where data changes frequently and the client needs to be immediately informed about these changes, such as in chat applications, live feeds, or real-time notifications.
The implementation of subscriptions involves:
- Client Subscription Request: The client sends a subscription request to the server, specifying the data it wants to receive updates about.
- Persistent Connection: The server establishes and maintains a persistent connection with the client.
- Event-Triggered Updates: When an event occurs that changes the subscribed data, the server pushes these updates to the client through the open connection.
Example:

Q14.

How do you handle pagination in GraphQL?

Senior

Offset-Based Pagination
- Concept: Similar to traditional pagination in SQL databases, it uses a combination of limit and offset parameters. limit specifies the number of items to return, and offset specifies the number of items to skip before starting to return the items.
- Implementation: The GraphQL query includes arguments for limit and offset. The server responds with the specified slice of the dataset.
- Example:
Cursor-Based Pagination
- Concept: More suited for real-time data and large datasets, it works by returning a pointer (cursor) to a specific item in the dataset. The client uses this cursor to request the next set of items.
- Implementation: The GraphQL query includes an argument for the cursor (often the ID of the last item retrieved) and a limit. The server responds with the data and a new cursor for the next query.
- Example:
Considerations:
- Offset-Based is simpler and more familiar but less efficient for large datasets as it requires counting over large numbers of rows.
- Cursor-Based is more complex but efficient for large, real-time datasets. It also helps in avoiding issues like skipping or double-fetching items in a changing dataset.

Q15.

How do you handle versioning and backward compatibility in GraphQL?

Senior

In GraphQL, versioning and backward compatibility are managed differently than in REST APIs, focusing on schema evolution rather than versioned endpoints:
- Additive Changes: Prefer adding new fields or types to the schema without removing existing ones, allowing clients to adapt gradually.
- Deprecation Strategy: Use the @deprecated directive to mark fields or types that will be removed or changed, guiding clients to updated usages.
- Field Aliases for Renaming: Introduce new fields with desired names and deprecate old ones, maintaining backward compatibility.
- Non-breaking Changes: Emphasize changes that don't break existing queries, like adding fields or new types.
- Documentation and Monitoring: Keep documentation up-to-date and monitor schema usage to inform deprecation and change strategies.
Example: If renaming a field getUser to getUserById, add getUserById as a new field and deprecate getUser:
- This approach ensures existing queries remain functional while guiding clients towards new schema elements.

Q16.

Explain GraphQL directives.

Senior

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Q17.

Explain the concept of batching and dataloading in GraphQL.

Senior

Q18.

How do you handle long-running queries or mutations in GraphQL?

Senior

Q19.

Discuss federated schemas and microservices architecture in GraphQL.

Senior

Q20.

How do you handle schema stitching with Apollo Server?

Senior

Q21.

How do you address the "N+1 query" problem in GraphQL?

Senior