More in TypeScript & Web APIs — page 4
What is the core difference between localStorage and sessionStorage?
WHAT IT TESTS: Tab isolation versus origin-wide persistence. ANSWER OUTLINE: sessionStorage dies with its tab; localStorage survives restarts and shares across tabs. Example: checkout form versus theme preference. RED FLAG: Claiming persistence time differs.
Write a generic CreateSetters<T> that maps properties to setter methods
This tests TypeScript mapped types with key remapping and template literals. It iterates keyof T, remaps via as set plus Capitalize of string and K, and types values as setter methods. Red flag: manually typing setters or omitting string and K in Capitalize.
Implement recursive DeepReadonly<T> for nested objects and arrays
Tests recursive mapped types and conditional type narrowing. A strong answer uses a conditional to split arrays into ReadonlyArray, objects into readonly mapped types, and leaves primitives untouched.
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.
Implement the built-in NonNullable<T> utility type from scratch
Tests conditional type distribution and union filtering. A strong answer uses distributive conditional types to map null and undefined to never while preserving other union members.
Implement MyParameters<T> using conditional types and infer
Tests if you can extract parameter types with infer. A strong answer matches T against a callable signature, uses infer to bind parameters into a tuple, and defaults to never for non-functions. Red flag: using any or indexing without inference.
Create TerminalStatus from Status using Exclude or Extract
This tests conditional type mechanics and whitelist versus blacklist thinking. Extract selects matches by assignability; Exclude removes others. Prefer Extract when the desired set is explicit. Red flag: claiming they are equivalent or only knowing Exclude.
Write a generic MakeOptional<T> mapped type without using Partial
This tests mapped type mechanics and the optionality modifier. A strong answer iterates over keyof T, adds the ? modifier, and preserves the original type via indexed access. A red flag is suggesting Object.assign or saying you would just use Partial.
Create a User type alias from a function return type
WHAT IT TESTS: TypeScript type-space introspection. ANSWER OUTLINE: Use typeof to grab the function type, then ReturnType to extract the return object shape. RED FLAG: Manually copying the object literal or confusing value-space and type-space typeof.
Describe a common use for Partial<T> and explain Required<T>
WHAT IT TESTS: Optional versus mandatory properties in TypeScript. ANSWER OUTLINE: Partial fits PATCH updates where only some fields arrive; Required strips optionality to enforce all properties. RED FLAG: Confusing with Pick or saying Partial allows nulls.
Explain Pick versus Omit with User examples
WHAT IT TESTS: Explicit selection versus type subtraction. ANSWER OUTLINE: Pick keeps listed keys; Omit drops them. Show User with id, name, email; derive Pick<User,"name"|"email"> and Omit<User,"id">. RED FLAG: Confusing them or calling one strictly safer.
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.
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.
Write a generic fetchJSON<T> wrapper and explain its type safety benefits
Tests preserving type info across async boundaries via generics. Outline: write fetchJSON<T> returning Promise<T>, note response.json() is any, and show T lets callers lock in the response shape for compile-time checks.
Create a generic State class with getState and setState
WHAT IT TESTS: Binding a generic class parameter so methods share one consistent type. ANSWER OUTLINE: Write class State<T> with private T, constructor(T), getState(): T, and setState(T). RED FLAG: Using any instead of T, erasing type safety.
Write a generic type-safe getProperty using keyof
Command of TypeScript generics and keyof for compile-time property access. Declare generic T, accept key as keyof T, and return T[K]. RED FLAG: Using any or string for the key, which erases type safety and allows invalid properties.
Explain generic constraints and provide a length-constrained generic example
WHAT IT TESTS: Your grasp of bounding generics to shapes without losing type safety. ANSWER OUTLINE: Explain that extends requires properties; show T extends { length: number }; give a generic function signature. RED FLAG: Using any or dropping extends.
Refactor a function using generics to accept any array type
It tests generic type parameters for preserving element types through a function boundary. Answer: declare T, accept T[], return T, and let the compiler infer the element type from the call site.
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.
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.