Full Stack Developer interview questions
59 questions asked in Full Stack Developer interviews, on end-to-end features, APIs and the product around them. They are built from thousands of real job ads. Under each one: what the interviewer is listening for, and the follow-up they may ask. For the recruiter call, see the HR screen questions.
Behavioral questions
Questions about how you work in this role. More in behavioral questions for any tech role.
-
1
Have you hit a problem that sat between the frontend and the backend, where each side thought it was the other's? What did you do?
All levels
Listening for Takes ownership across the boundary and gets both sides talking with data.
Follow-up Ask how they would stop it happening again.
-
2
Most fullstack people are stronger on one side. What's a piece of serious work you did on your weaker side?
Junior, Mid
Listening for Honest about the gap, a clear way of learning, and the result.
Follow-up Ask what they still find hard there.
-
3
Think of something you shipped quickly that came back to cause problems later. What happened?
All levels
Listening for Owns the shortcut, explains why it made sense then and how they cleaned it up.
Follow-up Ask how they decide when a shortcut is fine.
-
4
Tell me about a feature you shipped that users didn't use the way you expected. What did you change?
Mid, Senior
Listening for Looks at real data or feedback and changes course.
Follow-up Ask how they found out.
-
5
Have you ever got a product manager to change a plan because of something technical? What happened?
Mid, Senior
Listening for Explains the risk in product terms and offers an option, not just a no.
Follow-up Ask what the PM gave up in the end.
-
6
Tell me about an experiment or launch where the numbers told you something you didn't want to hear.
Mid, Senior
Listening for Accepts the data, digs into why and acts on it.
Follow-up Ask what they did next.
-
7
Tell me about a product launch where you owned the engineering side from start to finish. What almost went wrong?
Senior
Listening for Plans, spots risks early, coordinates people and handles the surprise.
Follow-up Ask what they would do differently.
Technical discussion questions
Talking through your experience and decisions, not a coding test. Grouped by topic.
TypeScript
-
1
Tell me about the hardest bug you've fixed. How did you find the cause?
All levels
Listening for A methodical search with logs, reproduction and narrowing down, not luck.
Follow-up Ask what would have caught it earlier.
-
2
What do you look for when you review someone's pull request?
All levels
Listening for Correctness, tests, readability and kindness in feedback, in a sensible order.
Follow-up Ask how they handle a review they strongly disagree with.
-
3
Which part of your tech stack do you know best, and what's tricky about it?
Junior
Listening for Real depth in one area and awareness of its sharp edges.
Follow-up Ask how they learned that tricky part.
React
-
4
A page in our app has become slow. How would you find out why?
All levels
Listening for Measures first and checks both sides: browser, network, API and database.
Follow-up Ask what they would do if the slow part is a query.
-
5
How does login work in an app you built, from the form in the browser to every API call after it?
All levels
Listening for Explains sessions or tokens, storage on the client and checks on the server.
Follow-up Ask what happens when the session expires mid-task.
-
6
In a React app you've worked on, how did you decide where state should live?
Junior, Mid
Listening for Clear reasons for local, shared and server state.
Follow-up Ask about a bug caused by state in the wrong place.
API design
-
7
How would you design the API for a simple to-do app? Just talk me through it.
All levels
Listening for Clear resources, methods, status codes, validation and how data is stored.
Follow-up Ask how they would add sharing between users.
System design
-
8
How would you design a simple app where users book a time slot with a coach? Talk me through the main parts.
Junior
Listening for Pages, API, data model and how to stop two people booking the same slot.
Follow-up Ask what changes if coaches are in different time zones.
-
9
How would you design a URL shortener that handles millions of requests a day?
Mid, Senior
Listening for Key generation, storage, caching for heavy reads and handling collisions.
Follow-up Ask what changes if links must expire.
-
10
Tell me about an architecture decision you made, and what you'd do differently now.
Senior
Listening for Context, options, trade-offs and honest hindsight.
Follow-up Ask how they got the team to agree at the time.
-
11
How would you design an activity feed for a product with a few million users?
Senior
Listening for Fan-out on write or read, storage, caching, pagination and handling very active users.
Follow-up Ask what changes if the feed must be real time.
-
12
Traffic to our product is expected to grow ten times in six months. What would you look at first?
Senior
Listening for Finds the real bottlenecks with load tests and data, plans for the database first, avoids rewriting everything.
Follow-up Ask what they would not change.
-
13
A mid-level engineer shares a design for a new feature that touches the web app, the API and the database. What would you look for?
Senior
Listening for Checks requirements, data model, API shape, failure cases and rollout, and gives feedback that teaches.
Follow-up Ask how they handle disagreement on the design.
Python
-
14
Which backend language have you used most, and what would you miss if you had to switch to another one?
Junior
Listening for Knows their main language beyond syntax: libraries, tooling and its weak spots.
Follow-up Ask what they would do to learn a new one quickly.
Distributed systems
-
15
Our app now runs on two servers instead of one, and some users get logged out at random. What might be going on?
Junior
Listening for Spots session state kept in one server's memory and suggests shared storage or stateless tokens.
Follow-up Ask what else might break the same way.
-
16
Our app sends a welcome email from a background job, and some users get it twice. Why might that happen?
Junior
Listening for Thinks of retries after a timeout, two workers picking the same job and missing idempotency, and how to dedupe.
Follow-up Ask how they would check which one it is.
-
17
Two people edit the same task at the same time. How would you keep both boards in sync?
Mid, Senior
Listening for Push updates, conflict handling such as last-write-wins or versions, and offline cases.
Follow-up Ask what the user sees when there is a conflict.
AWS
-
18
Have you deployed an app to the cloud yourself? What did you set up, and what surprised you?
Junior
Listening for Honest hands-on experience with hosting, config and secrets.
Follow-up Ask what they would do differently next time.
LLMs
-
19
Let's say we add a 'summarise this' button that calls an LLM. What would you need to handle on the frontend and the backend?
Junior, Mid
Listening for Streaming or loading states, timeouts, errors, cost, abuse limits and keeping keys on the server.
Follow-up Ask what the user sees when it fails.
-
20
Users say our AI feature sometimes gives wrong answers. How would you find out how often, and make it better?
Mid, Senior
Listening for Collects real cases, builds an eval set, measures before changing prompts or models.
Follow-up Ask how they would stop a fix from breaking other cases.
-
21
We want users to ask questions about their own documents inside our product. How would you build that?
Senior
Listening for Ingesting and chunking, embeddings, retrieval, citing sources and keeping each customer's data apart.
Follow-up Ask how they would measure answer quality.
Product engineering
-
22
Walk me through a feature you built across both the frontend and the backend. Where did most of the work end up?
Junior, Mid
Listening for Owns the whole path from UI to data and can explain the hard part.
Follow-up Ask what they would change about the split of work.
-
23
When you pick up a ticket for a new feature, what do you check before you start coding?
Junior
Listening for Understands the user goal, edge cases, design and API needs, and asks questions early.
Follow-up Ask about a time a question saved them rework.
-
24
How do you deal with technical debt when the team is under pressure to ship features?
Mid, Senior
Listening for Makes debt visible, ties it to delivery speed or risk and pays it down in small steps.
Follow-up Ask about debt they decided to leave.
-
25
A product manager asks for a feature that would take three weeks. How would you look for a smaller version to ship first?
Mid, Senior
Listening for Finds the core user problem, cuts scope with the PM and plans to learn from the first version.
Follow-up Ask how they would decide what comes next.
GraphQL
-
26
You've probably used REST or GraphQL between a web app and its backend. What did you like and dislike about it?
All levels
Listening for Real experience with fetching, caching and schema changes rather than slogans.
Follow-up Ask when they would pick the other one.
-
27
In a project that used GraphQL, what happened when someone changed the schema?
Junior
Listening for Knows how the client depends on the schema, generated types and breaking changes.
Follow-up Ask how the team caught breaks before release.
-
28
Our GraphQL page loads fine for one user but hammers the database under load. What would you look for?
Mid, Senior
Listening for N+1 queries, batching with data loaders, query cost limits and caching.
Follow-up Ask how they would spot it before release.
CI/CD
-
29
How do you decide what to test, and how much?
All levels
Listening for Risk-based choices across unit, integration and end-to-end tests.
Follow-up Ask about a bug that their tests missed.
-
30
Walk me through how your code gets from a merged pull request to production in your current team.
Junior, Mid
Listening for Knows the pipeline steps and checks in their own words.
Follow-up Ask which step catches the most problems.
-
31
How would you ship a big change to the checkout flow without one risky release day?
Mid, Senior
Listening for Feature flags, small merges, gradual rollout, metrics to watch and an easy way back.
Follow-up Ask what they would watch in the first hour.
AI agents
-
32
How do you use AI coding assistants in your work, and how do you check what they write?
Junior, Mid
Listening for Uses them with judgement and reviews the output as their own code.
Follow-up Ask about a time the suggestion was wrong.
-
33
Let's say our AI assistant can create calendar events for users. What could go wrong?
Junior
Listening for Wrong time zones or people, duplicates, acting without asking, and the need for confirmation and limits.
Follow-up Ask how they would let users undo it.
-
34
We want an AI assistant inside our product that can take actions for users, like creating tickets. How would you design it safely?
Mid, Senior
Listening for Tool calls through existing APIs with the user's permissions, confirmation for risky actions, logs and limits.
Follow-up Ask how they would test it.
-
35
Our AI agent feature works, but its cost per user has tripled in a month. How would you investigate?
Senior
Listening for Traces each run, finds long loops, large prompts or retries, adds limits and caching.
Follow-up Ask how they would weigh cost against quality.
Node.js
-
36
Our Node.js API slows down badly whenever one endpoint does heavy work. What would you do about it?
Senior
Listening for Knows the event loop is blocked by CPU work and moves it to workers or a queue.
Follow-up Ask how they would find which endpoint is the cause.
Kubernetes
-
37
Our small team runs everything on Kubernetes, and it takes a lot of our time. How would you decide whether to keep it?
Senior
Listening for Weighs the real cost of running it against managed options and team size.
Follow-up Ask what would make them keep it.
Observability
-
38
You get paged at night for one of your services. What do you do in the first ten minutes?
All levels
Listening for Checks impact, dashboards and recent deploys, communicates and mitigates before fixing.
Follow-up Ask when they would wake someone else up.
-
39
Tell me about a production incident you were part of. What did the team learn?
Mid, Senior
Listening for A clear timeline, their own part, root cause and follow-ups that were done.
Follow-up Ask what alert would have caught it sooner.
-
40
Some users say the app is broken, but your dashboards look fine. How would you find out what they're seeing?
Mid, Senior
Listening for Frontend error tracking, real user monitoring, segmenting by device or region.
Follow-up Ask what they would add so it shows up next time.
Java
-
41
If you've worked with Java or Kotlin on the backend, what did you find hard about it at first?
Junior, Mid
Listening for Honest learning story with concrete parts such as frameworks, build tools or types.
Follow-up Ask what finally made it click.
Go
-
42
Have you written code that did several things at once, like parallel API calls? What went wrong, or nearly did?
Junior, Mid
Listening for Understands async or concurrent work, error handling and limits on how many run at once.
Follow-up Ask how they tested it.
-
43
We're picking a language for a new backend service. What would you weigh, beyond what the team already knows?
Mid, Senior
Listening for Workload, ecosystem, hiring, operations and fit with existing services.
Follow-up Ask when they would choose something nobody on the team knows yet.
-
44
One of our Go services uses more and more memory over a day until it restarts. How would you track that down?
Mid, Senior
Listening for Uses metrics and profiles, compares snapshots, looks for leaks such as goroutines or caches.
Follow-up Ask how they would stop it reaching users meanwhile.
Experimentation
-
45
Product wants to A/B test a new signup flow. What would you need to build on the frontend and the backend?
All levels
Listening for Assignment that sticks per user, flags, event tracking on both sides and cleanup.
Follow-up Ask how they would know the numbers are right.
-
46
Have you shipped something behind a feature flag? How did it work, and when was the flag removed?
Junior
Listening for Understands flags for safe release and testing, and that old flags are debt.
Follow-up Ask what happens when nobody removes them.
-
47
An A/B test shows the new checkout lost sales, but the team thinks the data is wrong. How would you check?
Mid, Senior
Listening for Checks assignment, event loss, bots and pipeline bugs before trusting or rejecting results.
Follow-up Ask what they would do if the data is right.
Data pipelines
-
48
Product wants a daily report of active users from our app database. How would you build it without slowing the app down?
All levels
Listening for Read replica or a separate store, a scheduled job, clear metric definition.
Follow-up Ask what happens when the job fails one night.
-
49
Users want to upload a CSV with fifty thousand rows into our app. How would you handle that?
Junior, Mid
Listening for Background processing, validation with clear errors, batching and progress for the user.
Follow-up Ask what happens if row thirty thousand is bad.
-
50
The product team wants usage numbers that are no more than a minute old. How would you change our nightly reporting setup?
Senior
Listening for Event streaming versus micro-batches, cost, correctness and what really needs to be fresh.
Follow-up Ask how they would check that numbers match the old reports.
Kotlin
-
51
Our backend is in Kotlin, but most of the team knows TypeScript. How would you help the team work well across both?
Mid, Senior
Listening for Shared API contracts, pairing, good docs and choosing where each language fits.
Follow-up Ask what would make them move to one language.
-
52
We have an old Java service that nobody likes touching. Would you rewrite it in Kotlin, or improve it in place?
Senior
Listening for Weighs risk, team skills, interop and the real problem with the service, and prefers gradual change.
Follow-up Ask what would justify a rewrite.