Frontend Interview Prep — newest questions https://frontend-interview.com/ Free frontend interview prep: 265 worked JavaScript, TypeScript, React, system design & DSA questions, each pairing the problem signal with its technique. en-US Sun, 30 Aug 2026 08:25:33 GMT Function overloads vs generics vs unions https://frontend-interview.com/q/function-overloads-vs-generics-vs-unions.html https://frontend-interview.com/q/function-overloads-vs-generics-vs-unions.html When should you use function overloads, and when is a generic or a union parameter the better tool? as const and literal types vs enum https://frontend-interview.com/q/as-const-and-literal-types-vs-enum.html https://frontend-interview.com/q/as-const-and-literal-types-vs-enum.html What does `as const` do, and when would you prefer a literal union over a TypeScript enum? unknown vs any vs never https://frontend-interview.com/q/unknown-vs-any-vs-never.html https://frontend-interview.com/q/unknown-vs-any-vs-never.html Explain the difference between unknown, any, and never, and when to use each. Write your own mapped + conditional type https://frontend-interview.com/q/write-your-own-mapped-conditional-type.html https://frontend-interview.com/q/write-your-own-mapped-conditional-type.html Implement DeepReadonly and explain mapped types, keyof, and conditional types. Type guards & narrowing https://frontend-interview.com/q/type-guards-and-narrowing.html https://frontend-interview.com/q/type-guards-and-narrowing.html How does TS narrow types? Write a user-defined type guard, and show exhaustiveness with never. Model state with a discriminated union https://frontend-interview.com/q/model-state-with-a-discriminated-union.html https://frontend-interview.com/q/model-state-with-a-discriminated-union.html Model an async request's state so impossible states are unrepresentable, and TS narrows correctly. Partial, Pick, Omit, Record — and when https://frontend-interview.com/q/partial-pick-omit-record-and-when.html https://frontend-interview.com/q/partial-pick-omit-record-and-when.html Explain Partial, Required, Pick, Omit, and Record with a use case for each. Why generics? identity + a constrained generic https://frontend-interview.com/q/why-generics-identity-a-constrained-generic.html https://frontend-interview.com/q/why-generics-identity-a-constrained-generic.html Explain generics with a concrete example. Then constrain a generic so it only accepts objects with an `id`. Recursive types & type-safe object paths https://frontend-interview.com/q/recursive-types-and-type-safe-object-paths.html https://frontend-interview.com/q/recursive-types-and-type-safe-object-paths.html Write a recursive type. Then build a `Paths ` that produces the dotted key paths of a nested object, and a `Get ` that returns the type at a path. Branded (nominal) types over a structural system https://frontend-interview.com/q/branded-nominal-types-over-a-structural-system.html https://frontend-interview.com/q/branded-nominal-types-over-a-structural-system.html TypeScript is structural, so `UserId` and `OrderId` (both `string`) are interchangeable. How do you make them incompatible — and what's the cost? Variance & function parameter bivariance https://frontend-interview.com/q/variance-and-function-parameter-bivariance.html https://frontend-interview.com/q/variance-and-function-parameter-bivariance.html Explain covariance and contravariance for function types. Why are method parameters bivariant in TypeScript, and what does `strictFunctionTypes` change? Template literal types https://frontend-interview.com/q/template-literal-types.html https://frontend-interview.com/q/template-literal-types.html What are template literal types? Build an event-handler name type from a union of events, and show inferring a piece back out of a string pattern. keyof, typeof, and indexed access types https://frontend-interview.com/q/keyof-typeof-and-indexed-access-types.html https://frontend-interview.com/q/keyof-typeof-and-indexed-access-types.html Explain `keyof`, the `typeof` type operator, and indexed access types. Use them to write a fully type-safe property getter. The satisfies operator https://frontend-interview.com/q/the-satisfies-operator.html https://frontend-interview.com/q/the-satisfies-operator.html What problem does `satisfies` solve that neither a type annotation nor `as` does? Show a config object where it matters. Tuples, readonly arrays, and named elements https://frontend-interview.com/q/tuples-readonly-arrays-and-named-elements.html https://frontend-interview.com/q/tuples-readonly-arrays-and-named-elements.html How do you type a fixed-length tuple vs an array? Show named tuple members, a rest element, and `readonly` arrays — and why readonly matters. Type assertions: as, !, and why they're unsafe https://frontend-interview.com/q/type-assertions-as-and-why-they-re-unsafe.html https://frontend-interview.com/q/type-assertions-as-and-why-they-re-unsafe.html Explain type assertions (`as T`), the non-null assertion (`!`), and `as unknown as T`. Why are they escape hatches rather than conversions? Structural typing & excess-property checks https://frontend-interview.com/q/structural-typing-and-excess-property-checks.html https://frontend-interview.com/q/structural-typing-and-excess-property-checks.html TypeScript uses structural typing, not nominal. What does that mean — and why does an object literal with an extra field error when a variable with the same shape doesn't? type vs interface — when to use which? https://frontend-interview.com/q/type-vs-interface-when-to-use-which.html https://frontend-interview.com/q/type-vs-interface-when-to-use-which.html What's the difference between `type` and `interface`? When do you reach for each? Testing React components the user's way https://frontend-interview.com/q/testing-react-components-the-user-s-way.html https://frontend-interview.com/q/testing-react-components-the-user-s-way.html How should you query and assert on a React component with Testing Library? Why 'by role' over test IDs or class names? Code coverage: what it does and doesn't tell you https://frontend-interview.com/q/code-coverage-what-it-does-and-doesn-t-tell-you.html https://frontend-interview.com/q/code-coverage-what-it-does-and-doesn-t-tell-you.html What does code coverage measure, and why is '100% coverage' not the same as 'well tested'? Testing pure functions vs. side effects https://frontend-interview.com/q/testing-pure-functions-vs-side-effects.html https://frontend-interview.com/q/testing-pure-functions-vs-side-effects.html Why are pure functions the easiest thing to test, and how do you make code with side effects testable? Test doubles: stub, mock, spy, fake https://frontend-interview.com/q/test-doubles-stub-mock-spy-fake.html https://frontend-interview.com/q/test-doubles-stub-mock-spy-fake.html Define stub, spy, mock, and fake. When would you use each, and what's the risk of over-using them? Setup, teardown, and test isolation https://frontend-interview.com/q/setup-teardown-and-test-isolation.html https://frontend-interview.com/q/setup-teardown-and-test-isolation.html What do beforeEach/afterEach do, and why is shared state between tests dangerous? Assertions and matchers: toBe vs. toEqual https://frontend-interview.com/q/assertions-and-matchers-tobe-vs-toequal.html https://frontend-interview.com/q/assertions-and-matchers-tobe-vs-toequal.html What's the difference between toBe and toEqual? When do you reach for each, and what other matchers matter? Testing behavior, not implementation https://frontend-interview.com/q/testing-behavior-not-implementation.html https://frontend-interview.com/q/testing-behavior-not-implementation.html What does it mean to test behavior rather than implementation details? Why does it matter? What NOT to test, and testing ROI https://frontend-interview.com/q/what-not-to-test-and-testing-roi.html https://frontend-interview.com/q/what-not-to-test-and-testing-roi.html How do you decide what NOT to test? Explain testing ROI and the 'testing trophy' vs. the pyramid. What makes a good unit test https://frontend-interview.com/q/what-makes-a-good-unit-test.html https://frontend-interview.com/q/what-makes-a-good-unit-test.html What properties distinguish a good unit test from a bad one? What is the AAA structure? Test suite performance and parallelization https://frontend-interview.com/q/test-suite-performance-and-parallelization.html https://frontend-interview.com/q/test-suite-performance-and-parallelization.html A test suite takes 20 minutes and blocks every PR. How do you make it fast without losing confidence? Visual regression testing https://frontend-interview.com/q/visual-regression-testing.html https://frontend-interview.com/q/visual-regression-testing.html What is visual regression testing, what does it catch that DOM tests miss, and what makes it hard? Snapshot testing: value, pitfalls, and rot https://frontend-interview.com/q/snapshot-testing-value-pitfalls-and-rot.html https://frontend-interview.com/q/snapshot-testing-value-pitfalls-and-rot.html What is snapshot testing good and bad at? How do snapshots 'rot', and how do you use them responsibly?