TYPESCRIPT · MEDIUM CODEX
TypeScript Interview Questions — Medium
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50 Theory Questions7 Implementation Folios5 Free Model Answers
THEORY QUESTIONS & SOLUTIONSShowing 5 of 50 questions
Q1Compare Index Signatures and Mapped Types.
- Index Signatures (
[key: string]: any): Define arbitrary properties on objects. Flexible, but cannot restrict keys to a specific finite union of literals. - Mapped Types (
[K in Keys]: T): Use iteration to construct an object type based on a source union of keys. Allow dynamic property construction with full type safety.
Q2What are Discriminated Unions (Tagged Unions)? Why are they powerful?
- Definition: A union of object types where each object has a common literal property (the "discriminated" tag).
type Shape = { kind: "circle"; radius: number } | { kind: "square"; side: number }; - Power: Enables the compiler to perform complete, automatic type narrowing inside conditional or
switchblocks based on checking the literal tag.
Q3How do Generics with defaults work?
- Definition: Specifying a default fallback type inside generic parameters if no explicit type argument is passed during invocation.
interface Config<T = string> { value: T; }
Q4Compare `extends` in Type Aliases vs. Interfaces.
- Interfaces:
interface A extends B {}represents static OOP-style inheritance. If properties overlap with incompatible types, it throws a compile-time error. - Type Aliases: Used in Conditional Types (
T extends U ? X : Y), checking if a type is assignable to/compatible with another structure. Highly dynamic.
Q5Explain Mapped Type modifiers (e.g., `-readonly` or `-?`).
- Prefix Modifiers (
+or-): Modify existing property constraints during mapping.-readonly: Removesreadonlyfrom all properties.-?: Removes optionality, making all properties strictly required.
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