Question 1
Difficulty: easy
How do you use TypeScript to prevent bugs in a front-end or back-end codebase?
Sample answer
I use TypeScript as a design tool, not just a type checker. At the start of a feature, I try to model the important domain objects clearly with interfaces, type aliases, and discriminated unions so the code communicates intent. That usually catches issues earlier than runtime testing alone. I also rely on strict compiler settings, especially strictNullChecks and noImplicitAny, because they force me to deal with edge cases instead of letting them hide. In day-to-day work, I pay attention to API boundaries, because that is where many bugs enter the system. I validate external data, type the response carefully, and avoid assuming third-party services are perfectly shaped. When something is genuinely dynamic, I use runtime validation with TypeScript so the types reflect reality. The result is fewer surprises in production and code that is easier for the team to maintain and review.
Question 2
Difficulty: medium
How do you decide when to use interfaces, type aliases, unions, or generics in TypeScript?
Sample answer
I usually choose based on the shape of the problem and the amount of flexibility I need. If I am describing an object contract that may be extended or implemented across the codebase, I often start with an interface because it reads naturally and is easy to grow over time. If I need a more expressive type, such as a union, intersection, mapped type, or a function signature with composition, I lean toward a type alias. For example, unions are great when a value can only be one of a few known states, especially with discriminated unions for UI or request status flows. Generics come in when I want reusable abstractions without losing specificity, like a typed API wrapper or a table component. I try not to over-engineer the type system, though. The goal is to make the code safer and clearer, not to create something so abstract that no one wants to touch it later.
Question 3
Difficulty: medium
Tell me about a time you had to debug a difficult TypeScript issue.
Sample answer
In one project, we had a component that worked fine in development but failed in production when data came from an API with a slightly different shape than the mock data. The issue was not obvious because the code compiled cleanly, and the failing path only appeared when one field was missing. I traced it by checking the assumptions in the type definitions and comparing them against the real payload. The problem was that we had typed the API response too optimistically, so the code assumed values were always present. I fixed it by introducing a stricter response model, adding runtime validation at the boundary, and handling the nullable cases explicitly in the UI. I also added a small set of tests around the transformation logic. The bigger lesson for me was that TypeScript helps most when the types match reality, not when they only describe what we hope the data looks like.
Question 4
Difficulty: medium
How do you handle a situation where the TypeScript type system feels too strict or gets in the way of delivery?
Sample answer
When the type system starts fighting the implementation, I treat that as a signal to step back and check whether the design is too rigid or the abstraction is wrong. I do not immediately weaken the types with any or broad casts unless it is truly temporary and isolated. First I ask whether the code is mixing multiple responsibilities, because that is often why the types become awkward. Breaking the logic into smaller functions or introducing a clearer domain model usually resolves the friction. If external data is involved, I separate validation from business logic so the core code stays typed and predictable. In some cases, I will use a narrow, well-documented escape hatch, but only at a boundary and never in the middle of the application logic. My goal is to keep the developer experience practical while still preserving the benefits of type safety, maintainability, and readable code.
Question 5
Difficulty: hard
How would you build a strongly typed API integration in TypeScript?
Sample answer
I would start by defining the expected request and response shapes as close to the API boundary as possible. If the API is internal, I would align with the backend team on a shared contract or at least shared schema conventions. Then I would create a small client layer that handles fetching, parsing, and validation, rather than scattering fetch calls throughout the app. For the response, I would not trust the network blindly. Even with TypeScript, I would validate the payload at runtime using a schema or custom guard before passing the data into the business layer. That protects us from backend changes, partial outages, or malformed data. I also like to type loading, success, and error states explicitly so components cannot ignore them. For example, a discriminated union makes state handling much more reliable. This approach keeps the code safe, testable, and easier to refactor when the API evolves.
Question 6
Difficulty: medium
Describe a time when you improved the maintainability of a TypeScript codebase.
Sample answer
On one team, the codebase had grown quickly and a lot of the type logic was duplicated across features. Different developers were defining slightly different versions of the same data shape, which created inconsistencies and made refactoring painful. I proposed consolidating the shared domain types and creating a small set of reusable utilities for common patterns like API responses, form state, and error handling. I also replaced a few overly broad types with narrower models, so components only received the data they actually needed. That made the code easier to understand and reduced accidental coupling. To support the change, I worked with the team to update the lint rules and added examples in the documentation so people could follow the new patterns. After that, onboarding became smoother, and changes to one part of the system stopped causing surprises elsewhere. For me, maintainability is about making the safest path the easiest path for the whole team.
Question 7
Difficulty: easy
How do you approach typing React components in TypeScript?
Sample answer
I try to keep React component types simple and practical. For props, I define clear interfaces or type aliases that describe exactly what the component needs, and I avoid passing large, generic objects when a smaller shape will do. If the component is reusable, I pay close attention to optional fields, callbacks, and children so the API is obvious to other developers. For state, I prefer explicit types, especially when the state can exist in multiple modes, such as loading, empty, error, or ready. In those cases, a union type often works better than a loose object with many optional properties. I also type event handlers carefully, because that is where small mistakes can become runtime issues. When working with hooks, I make sure the generic types are inferred correctly or supplied where necessary. My overall goal is to create components that are both easy to use and hard to misuse, without making the props verbose or awkward.
Question 8
Difficulty: medium
How do you ensure TypeScript types stay aligned with a changing product or API?
Sample answer
I try to treat types as living documentation, so I keep them close to the source of truth and update them as the product evolves. When an API changes, I do not just patch the compiler errors. I look at why the shape changed and whether the code should adapt in one place or across the system. If possible, I prefer schema-driven contracts or shared models so changes are less likely to drift between teams. I also use tests around transformation logic because those tests make type updates safer and show where assumptions are baked into the code. On the product side, I like to name states and domain concepts clearly, which helps when requirements shift. If a field becomes optional or a workflow gains another status, I update the union or interface explicitly instead of hiding the change with a cast. That discipline keeps the codebase honest and prevents small changes from turning into hard-to-find bugs later.
Question 9
Difficulty: easy
What is your approach to code reviews for TypeScript pull requests?
Sample answer
In TypeScript reviews, I focus on both correctness and readability. I look for places where the types reflect reality versus places where someone may have forced the compiler to stay quiet. If I see an any, a cast, or a broad object type, I ask whether there is a narrower and safer way to express the same idea. I also check whether the code communicates intent clearly through naming and type structure, because strong types should make the code easier to understand, not harder. Another thing I watch for is overcomplication. Sometimes someone creates a very clever generic solution when a simple interface would be easier to maintain. I try to give feedback that balances safety with practicality, especially if the team is moving quickly. Good TypeScript code review is not about making everything perfect; it is about reducing future risk, keeping the API boundaries clean, and helping the team build confidence in the code they ship.
Question 10
Difficulty: hard
How would you handle a production bug caused by a type mismatch that TypeScript did not catch?
Sample answer
First, I would treat it as a systems problem rather than just a typing problem. I would identify where the mismatch entered the application, whether it came from an API, user input, a third-party library, or a transformation step inside our code. Then I would reproduce the issue with real data if possible and add logging or tracing around the failing path so we can confirm the root cause. If TypeScript did not catch it, that usually means the boundary was typed too loosely or runtime validation was missing. I would fix that by tightening the types and adding a runtime check where the data crosses into trusted code. I would also add a regression test so the same mismatch fails earlier next time. After the immediate fix, I would review whether similar risks exist elsewhere in the codebase. My goal in those situations is not just to patch the bug, but to turn it into a stronger guardrail for the team going forward.
