Advanced interview questions in TypeScript & Web APIs
Create a generic getProperty using generics and keyof
Whether you can constrain a generic key with keyof and return the exact property type. Use T for the object and K extends keyof T for the key, returning T[K].
What is a type predicate? Write a custom type guard for User.
Tests if you know a type predicate is a compile-time narrowing hint requiring runtime validation. Great answers define parameter is Type, validate every User property, and demonstrate narrowing. Red flag: omitting runtime checks or using as casting.
Offload CPU-intensive work to a Web Worker and explain communication.
This tests main-thread blocking and worker messaging. A strong answer covers: new Worker(url), moving logic to a worker file, sending data via postMessage(), and receiving results via onmessage. A red flag is suggesting DOM use or shared memory from workers.
Explain Shadow DOM, encapsulation, and event propagation
Tests Web Components encapsulation and event retargeting. A strong answer covers shadow root/host/boundary, CSS scoping, open versus closed modes, and retargeting where events appear to come from the host element.
Build a typesafe ApiRoute type using template literal types
Tests type-level string composition with template literals. Good answer: define a Resource union, interpolate it into /api/v1/ paths, then union with the /{id} variant. Red flag: suggesting runtime regex or plain string instead of literal types.
How would you model fetchItem's return type with generics and conditional types?
Define base and extended Item, return T extends true ? ExtendedItem : BaseItem, and overload the plain boolean case.
Difference between abstract class and interface, with scenario
Tests if you know when shared state or constructor logic justifies single inheritance. A strong answer contrasts erased interfaces with base classes, then names a scenario requiring enforced initialization.
What is the polymorphic this type in TypeScript?
This tests polymorphic this for type-safe fluent APIs. A strong answer defines this as the current instance type, implements CSSBuilder methods that return this for chaining, and notes subclass preservation.
Implementing the Singleton pattern in TypeScript
Private constructor blocks new, a static private instance field caches it, static getInstance lazily creates and returns the one instance.
How would you use DocumentFragment to optimize adding 1,000 list items?
Tests DOM reflow/repaint costs and off-DOM batching. A strong answer: create a DocumentFragment, build the 1,000 nodes off-DOM, then append once to trigger a single reflow. Red flag: claiming it saves memory or confusing it with innerHTML batching.
Implement IntersectionObserver in TypeScript for lazy-loading images
Tests precise DOM typing and observer lifecycle. A strong answer types the callback as receiving IntersectionObserverEntry[], narrows entry.target to HTMLImageElement, swaps data-src to src, and calls unobserve.
Write a generic TypeScript handler for mixed form inputs?
Tests TypeScript DOM narrowing. Strong answer: union-type ChangeEvent, narrow target with instanceof or tag checks, branch on element.type to read .checked for checkboxes or .value otherwise. Red flag: casting to any or assuming all inputs use .value.
How would you prevent search-as-you-type race conditions with AbortController?
Abort the previous request before each new keystroke, pass the fresh signal into fetch, and ignore the AbortError.
Implement inSequence(tasks) to execute promise-returning functions sequentially
Tests async/await control flow versus Promise.all parallelism. A strong answer uses a for-of loop with await inside an async function, accumulates results in order, and stops cleanly on rejection. Red flag: suggesting Promise.all or forEach with await.
How do you cancel pending fetches using AbortController?
Keep one AbortController, abort before each fetch, pass its signal, and swallow AbortError.
When does fetch trigger a CORS preflight, and what POST is complex?
Tests whether you know the simple-request boundary. A strong answer names the three safe POST content-types and gives a cross-origin POST with application/json plus a custom header like Authorization.
Download a large file via fetch and track ReadableStream progress
It tests streaming I/O and backpressure beyond Promise-based fetch. Acquire a reader from response.body, loop read() until done, sum chunk lengths against Content-Length, and emit progress.
Implement MyReturnType<T> using conditional types and infer
This tests type-level pattern matching with conditional types and infer. Answer uses T extends (...args: any[]) => infer R ? R : never, noting infer captures return slot, non-functions yield never. Red flag: conflating runtime values with compile-time types.
Implement a generic PickByType<T, U> utility type
Tests mapped-type key filtering via conditional types and as remapping. Great answer: [K in keyof T as T[K] extends U ? K : never]: T[K]. Red flag: mapping all keys and setting values to never, which preserves keys instead of removing them.
Write a generic PickByValue<T, V> type
Tests conditional types and mapped-type key remapping. Great answer: K in keyof T as T[K] extends V ? K : never with value T[K]. Red flag: writing V extends T[K] instead, which reverses the assignability check and includes wrong keys.
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