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What is GraphQL, and how does it differ from a traditional REST API in terms of how clients request data? What is the difference between a GraphQL query variable and hardcoding a value directly inside the query string? How would you set up a basic GraphQL server in Node.js using Apollo Server, defining a schema and resolvers? What is GraphQL introspection, and why might you want to disable it in a production environment? What is the purpose of GraphQL fragments, and how do they help avoid duplicating field selections across multiple queries? How would you handle file uploads in a GraphQL API, given GraphQL's standard JSON-based transport doesn't naturally support binary file data? What is the purpose of GraphQL's non-null (!) type modifier, and what happens if a resolver for a non-null field returns null unexpectedly? What is the difference between a GraphQL scalar type and an object type, and how would you define a custom scalar (like DateTime) in a Node.js GraphQL server? What is the N+1 query problem in GraphQL specifically, and how does DataLoader solve it? How would you implement authentication and authorization in a GraphQL API in Node.js, given there's typically only a single HTTP endpoint?
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What is the difference between a GraphQL Query, Mutation, and Subscription operation type?

Intermediate

Answer

A Query fetches data without modifying anything, analogous to a REST GET request. A Mutation performs a write operation that changes data (creating, updating, or deleting a resource), analogous to a REST POST/PUT/DELETE. A Subscription establishes a persistent connection (typically over WebSockets) that pushes real-time updates to the client whenever a specified event occurs on the server, with no direct REST equivalent, since REST has no standard mechanism for server-initiated updates.
type Query { posts: [Post!]! }
type Mutation { createPost(title: String!, body: String!): Post! }
type Subscription { postCreated: Post! }

# Client subscribing to real-time updates
subscription { postCreated { id title } }
Real-world example A live-blogging application uses a Query to load initial posts on page load, a Mutation for the author to publish a new post, and a Subscription that pushes each newly published post to all currently connected readers' browsers in real time without them needing to refresh.

Common follow-ups: What transport mechanism does a GraphQL Subscription typically use under the hood to push real-time data to clients?;How would you handle authorization differently for a Mutation compared to a read-only Query?

WebSockets & Real-Time Communication;RESTful API Design with Express
All 15 GraphQL questions