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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 the difference between a GraphQL Query, Mutation, and Subscription operation type? 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 GraphQL, and how does it differ from a traditional REST API in terms of how clients request data?

Beginner

Answer

GraphQL is a query language and runtime for APIs where the client specifies exactly which fields it needs in a single request, and the server returns precisely that shape of data -- unlike REST, where each endpoint returns a fixed data shape (often requiring multiple requests to different endpoints to gather related data, or over-fetching unnecessary fields from a single endpoint), GraphQL exposes a single endpoint with a strongly-typed schema, letting clients compose exactly the query they need.
# GraphQL query: client specifies exactly the fields it wants
query {
  user(id: "123") {
    name
    email
    orders { id total }
  }
}

# A single request returns exactly this nested shape, no more, no less
Real-world example A mobile app with limited bandwidth switches a screen from three separate REST API calls (user profile, orders, and preferences) to a single GraphQL query requesting exactly the fields needed for that screen, reducing both the number of round trips and the amount of unused data transferred.

Common follow-ups: What's the specific problem of 'over-fetching' and 'under-fetching' in REST that GraphQL is designed to solve?;Does GraphQL replace REST entirely, or are there scenarios where REST is still the better choice?

RESTful API Design with Express;HTTP & Web Servers
All 15 GraphQL questions