DevAcademy

1. What is an index signature used for?

Typing an arrayTyping an object whose keys aren’t known ahead of time but share a common value typeAdding a numeric index to a stringSorting object keys

2. What does Record<string, number> describe?

An array of numbersAn object with string keys and number valuesA tuple of a string and a numberA function type

3. When does TypeScript perform excess property checks?

Always, for every assignmentWhen passing an object literal directly to a typed parameterOnly for arraysNever

4. What type does an optional property implicitly include?

nullundefinedneverany

5. What happens if you try to reassign a readonly property after creation?

It silently succeedsTypeScript reports a compile errorIt throws at runtime automaticallyNothing, readonly has no effect

6. Does readonly provide true runtime immutability like Object.freeze()?

Yes, identical behaviorNo — it is a compile-time-only checkOnly for arraysOnly for primitives

7. Does a type assertion convert a value at runtime?

Yes, it converts the valueNo — it only changes how the compiler treats the typeOnly for numbersOnly in strict mode

8. What does the trailing ! (non-null assertion) tell the compiler?

The value is definitely an arrayThe value is definitely not null or undefinedThe value should be loggedThe value is a constant

9. What happens if a type assertion turns out to be wrong?

TypeScript catches it at compile time regardlessIt surfaces as a runtime error instead, since the check was skippedThe code simply won’t compileNothing, TypeScript silently corrects it

10. What does the parameter property shorthand (constructor(public name: string)) do?

Nothing extra, it’s just a regular parameterDeclares and initializes a class property from the constructor parameter automaticallyMakes the parameter optionalOnly works with private properties

11. What must a subclass constructor call before using this, if the parent has its own constructor?

this()super()extend()init()

12. Can an abstract class be instantiated directly with new?

Yes, alwaysNo — it must be extended by a concrete subclass firstOnly if it has no abstract methodsOnly in strict mode

13. Which access modifier is the default when none is specified?

privateprotectedpublicreadonly

14. Can a protected member be accessed from a subclass?

No, neverYes, from the class and its subclassesOnly from outside codeOnly if also public

15. What is the key difference between TypeScript’s private and native # private fields?

There is no differenceNative # fields are enforced at runtime; TypeScript’s private is compile-time onlyprivate is faster# fields only work in interfaces

16. Which feature is unique to interface and not available with type?

Extending another shapeDeclaration mergingDescribing object shapesBeing used as a function parameter type

17. Can a type alias represent a union like string | number?

No, only interface canYesOnly with extendsOnly for primitives

18. What happens if you declare two interfaces with the same name?

The second one silently overrides the firstIt’s a compile errorThey automatically merge into one combined interfaceOnly the first one is used

19. What problem do generics solve?

They make code run fasterThey let reusable code stay type-safe instead of using any or duplicating logic per typeThey remove the need for interfacesThey only work with arrays

20. In function firstElement<T>(arr: T[]): T, how is T determined?

It must always be manually specifiedTypeScript infers it from the argument passed at the call siteIt is always "any"It is fixed at "string"

21. Can interfaces and classes be generic, not just functions?

No, only functionsYes, both interfaces and classes can be genericOnly classes canOnly interfaces can

22. Why does an unconstrained generic <T> not allow accessing value.length?

It’s a TypeScript bugT could be any type, and TypeScript can’t guarantee every type has a length propertylength is a reserved wordGenerics never allow property access

23. What does <T extends HasLength> do?

Makes T optionalRestricts T to types that satisfy the HasLength shapeConverts T into HasLengthRemoves type checking for T

24. What does K extends keyof T typically guarantee in a function signature?

K is always a stringK is restricted to one of the actual property keys of TK must be a numberNothing, it’s just documentation

25. What does narrowing mean in TypeScript?

Reducing a file’s sizeDeducing a more specific type for a value within a conditional block, based on a runtime checkConverting a type to anyRemoving unused imports

26. Which operator narrows a union based on whether an object has a specific property?

typeofinstanceofinas

27. What makes a discriminated union safe and exhaustive to narrow?

Using any for all membersA shared literal "tag" property that TypeScript can check to identify the exact shapeAvoiding interfaces entirelyUsing classes instead of objects

TypeScript Intermediate Practice

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