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1. What does Partial<User> do?

Makes every property of User requiredMakes every property of User optionalRemoves half of User’s propertiesCreates an array of User

2. What is the difference between Pick and Omit?

They do the same thingPick keeps only the listed keys, Omit removes the listed keysPick works on arrays, Omit works on objectsOmit is deprecated

3. What does ReturnType<typeof someFunction> give you?

The function’s parameter typesThe type of the value the function returnsThe function itselfA boolean indicating if the function is async

4. What does a mapped type do, conceptually?

It creates a JavaScript Map objectIt transforms every property of an existing type using the same rule, like map() for typesIt merges two unrelated typesIt converts a type into JSON

5. What does the -? modifier do inside a mapped type?

Makes every property optionalRemoves the optional modifier, making properties requiredDeletes the property entirelyMakes the property readonly

6. What does the as clause inside a mapped type allow?

Type assertionsRenaming each key as it’s mapped overCasting the whole type to anyAdding comments to types

7. What does T extends U ? X : Y mean?

T must always extend U at runtimeIf T is assignable to U, resolve to type X, otherwise resolve to type YIt merges T and UIt’s invalid TypeScript syntax

8. What does the infer keyword do inside a conditional type?

Asserts a value is not nullIntroduces a new type variable to capture part of a matched typeForces a type to be inferred as anyOnly works with arrays

9. What happens when a conditional type is applied to a union type?

It errors immediatelyIt automatically distributes over each member of the unionIt only applies to the first memberIt converts the union into an intersection

10. What does a function return type like value is User indicate?

It always returns a User objectIt’s a type predicate — if it returns true, TypeScript narrows value to UserIt throws if value is not a UserIt converts value into a User

11. Why are custom type guards useful for validating unknown data?

They automatically fetch data from an APIThey let you safely check and narrow unknown values before trusting their shapeThey remove the need for interfacesThey only work with arrays

12. Does TypeScript verify that a type guard’s implementation is actually correct?

Yes, it runs the logic and checks the resultNo — it trusts the declared return type, so an incorrect guard compiles fineOnly in strict modeOnly for guards using instanceof

13. Does TypeScript use a different import/export syntax from JavaScript?

Yes, completely differentNo — it uses the same ES module syntax, with the addition of type-only importsOnly for default exportsOnly in .tsx files

14. What does import type guarantee?

The import runs before other codeThe import is completely erased from the compiled JavaScript outputThe imported value becomes readonlyIt imports a namespace instead of a module

15. Should new TypeScript projects use namespaces or ES modules?

Namespaces, they are fasterES modules — namespaces are a legacy featureBoth equallyNeither, use CommonJS require only

16. What does a .d.ts file contain?

Only implementation code, no typesOnly type information, no runtime implementationCSS stylesTest cases

17. What is DefinitelyTyped?

A TypeScript compiler flagA community repository of type definitions for JavaScript packages, published under @typesA linting toolA build tool

18. What does declare const APP_VERSION: string do?

Creates the variable at runtimeTells TypeScript a value named APP_VERSION exists at runtime, without TypeScript needing to define itImports a moduleDeclares a new class

19. What does a decorator let you do?

Only add comments to codeObserve, modify, or replace a class, method, or property at definition timeCompile TypeScript fasterConvert TypeScript to JavaScript

20. What is a decorator factory?

A special kind of classA function that returns a decorator, allowing configuration options to be passed inA build tool for decoratorsA decorator that only works on factories

21. Which frameworks commonly use decorators as part of their API?

Only vanilla Node.jsAngular and NestJS, among othersOnly testing frameworksNone, decorators are unused in practice

22. What does the target option control?

Which files are compiledWhich JavaScript version the compiled output targetsThe output folderWhether strict mode is enabled

23. Is noUncheckedIndexedAccess included in strict mode by default?

Yes, alwaysNo — it must be enabled separatelyOnly in newer projectsIt replaces strict mode

24. What does skipLibCheck do?

Disables all type checkingSkips type-checking .d.ts declaration files, speeding up compilationSkips checking your own source filesRemoves the need for a tsconfig.json

25. Why start a project with strict mode enabled from the beginning?

It compiles fasterRetrofitting strict mode onto a large codebase later requires fixing many errors at onceIt’s required by npmIt disables type checking

26. Why should data from an external API be validated, not just typed?

TypeScript automatically validates API responsesTypeScript types are erased at runtime and don’t check that real data actually matchesValidation is only needed for arraysfetch() automatically validates JSON

27. What is the benefit of modeling state as a discriminated union instead of many optional fields?

It uses less memoryIt makes invalid or contradictory states genuinely unrepresentable, not just discouragedIt compiles to smaller JavaScriptIt removes the need for interfaces

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