To interview a full stack developer, test both client and server competence plus the seam between them: API design, state management, authentication flow, and database modeling. This set probes how a candidate moves a feature end to end, where they place logic, and how they reason about contracts between frontend and backend.
Structure a full stack interview around a single feature traced from UI to database, so you can see how the candidate connects layers rather than treating frontend and backend in isolation. Score breadth plus enough depth in at least one layer.
Walk me through building a feature end to end, from a button click to a row in the database and back.
What to look for: Coherent flow across request, validation, persistence, and response; clear ownership of error handling and loading states at each layer.
How do you design an API contract so the frontend and backend can develop in parallel?
What to look for: Agreeing on a schema or OpenAPI spec first, mocking endpoints, versioning, and treating the contract as the source of truth.
Where do you put validation: client, server, or both, and why?
What to look for: Knows client validation is UX only and the server is the trust boundary, never relying on the client for security.
How do you manage authentication and sessions across a single-page app and an API?
What to look for: Understands tokens vs cookies, httpOnly and SameSite, refresh flows, CSRF, and where secrets live.
You have an N+1 query problem on a list endpoint. How do you find and fix it?
What to look for: Inspects query logs or an ORM profiler, uses eager loading or a join or batching, and verifies the count dropped.
How do you keep frontend and backend types in sync to avoid runtime shape mismatches?
What to look for: Shared types, codegen from the schema, or end-to-end typing; awareness that JSON over the wire is untyped.
Describe how you'd implement optimistic UI updates and roll back on failure.
What to look for: Updates local state immediately, reconciles with the server response, and handles the error path cleanly without leaving stale UI.
How do you structure a project so business logic isn't tangled into UI components or route handlers?
What to look for: Separates concerns into a service or domain layer, keeps controllers and components thin, and makes logic testable independent of the framework.
When you're stronger on one side of the stack, how do you keep your weaker side from becoming a liability?
What to look for: Honest self-assessment, deliberate skill-building, and knowing when to pull in a specialist.
Tell me about a time a frontend and backend assumption diverged and caused a bug.
What to look for: Recognizes the contract gap, improves the handoff, and adds a test or schema check to prevent recurrence.
Describe a feature you shipped solo across the whole stack. What was hardest?
What to look for: Ownership end to end, realistic account of the integration pain, and pragmatic scoping.
How do you decide when a piece of logic belongs on the client versus the server?
What to look for: Reasons about trust, performance, data sensitivity, and duplication, placing security-critical logic server-side by default.
A page loads slowly and you don't know if it's the query, the API, or the render. How do you diagnose it?
What to look for: Measures each layer with browser devtools, server timing, and DB logs before guessing, then fixes the proven bottleneck.
Product wants a new field shown everywhere by Friday. How do you plan the work across the stack?
What to look for: Migration, API change, type updates, UI, and tests sequenced sanely, with backward compatibility considered.
You need to add real-time updates to an existing request-response app. What are your options?
What to look for: Weighs polling, SSE, and WebSockets against complexity and scale, and picks the simplest sufficient option.
An API change you need would break the existing mobile client. How do you roll it out?
What to look for: Versions the endpoint or adds the field additively, supports both shapes during transition, and coordinates the client cutover.
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