Use this guide to interview a Fullstack Engineer — someone expected to move comfortably between the browser, the API layer, and the database. Pick eight to twelve questions per session rather than running the whole list, keep the same set across candidates so scores stay comparable, and rate answers against the competencies below instead of overall impressions. Swap in {{Your product or domain}} wherever it makes a question concrete.

Frontend engineering

• Walk me through how you decide between rendering a page on the server and rendering it on the client. What signals drive the choice?
• How do you manage state shared across distant parts of a UI? When does local state stop being enough?
• Describe a time you improved the perceived performance of a page. What did you measure before and after?
• How do you make a complex form accessible? Name concrete techniques, not just "follow the guidelines".
• What happens in the browser between typing a URL and seeing an interactive page? Go as deep as you can.
• How do you keep a design system consistent when several engineers ship UI every day?

Backend and API design

• Design an API for {{A core feature of your product}}. Walk me through resources, verbs, and error handling.
• How do you version an API that external clients already depend on?
• When would you reach for a message queue instead of a synchronous call, and what new failure modes does that introduce?
• How do you handle authentication and authorization differently? Where has this distinction bitten you?
• Tell me about an endpoint you made idempotent. Why did it matter?
• How do you protect an API from a client that suddenly sends ten times the expected traffic?

What good looks like: strong candidates reason about contracts and failure modes, not just happy paths. They bring up idempotency, pagination, and backwards compatibility unprompted, and they can describe a real trade-off they made — for example, accepting eventual consistency to keep an endpoint fast — along with the consequences it had later.

Databases and data modeling

• How do you decide between a relational database and a document store for a new feature?
• A query that was fast last month is slow today. Walk me through your diagnosis step by step.
• What is an N+1 query, and how have you eliminated one in practice?
• How do you run a schema migration on a table with millions of rows without downtime?
• When have you deliberately denormalized data, and what did it cost you later?

System design and architecture

• Design {{A feature such as notifications or file uploads}} end to end: client, API, storage, and background work.
• Where do you place caching in a fullstack application, and how do you invalidate each layer?
• How would you evolve a single-server deployment into one that survives a region outage? What changes first?
• Tell me about a time you chose boring technology over the exciting option — or the reverse. How did the choice age?
• What belongs in a background job versus the request cycle, and how do you handle a job that fails halfway through?

What good looks like: look for candidates who ask about scale and constraints before drawing boxes, name the weaknesses of their own design, and can zoom from the browser down to the database inside a single answer — that vertical range is the core of the role.

Debugging and quality

• A user reports that "the app is broken" with no other detail. What do you do in the first ten minutes?
• What do you test at each layer of the stack, and what do you deliberately leave untested?
• Tell me about the hardest production bug you have chased across the stack. What made it hard?
• How do you decide a pull request is safe to approve when it touches code you did not write?

Collaboration and delivery

• How do you slice a large feature so it ships in small, safe increments?
• Describe a disagreement with a designer or product manager about feasibility. How did it resolve?
• When have you pushed back on a deadline, and how did you make the case?
• What do you do when you are blocked on another team for something critical?