TypeScript

Question 1

What is the difference between ‘type’ and ‘interface’ in TypeScript?

Answer 1

Both are used to define shapes of objects, but ‘interface’ is extendable and better for OOP patterns, while ‘type’ is more versatile for unions and intersections.

Question 2

How does TypeScript improve code safety in frontend/backend projects?

Answer 2

By providing static typing, preventing runtime errors, and offering better IDE support with autocompletion and refactoring.

Question 3

What are TypeScript’s ‘utility types’ and give an example?

Answer 3

Utility types like Partial<T>, Pick<T, K>, Omit<T, K> help transform existing types. E.g., Partial<T> makes all properties optional.</T></T>

Question 4

Explain ‘type inference’ in TypeScript.

Answer 4

TypeScript automatically deduces types from assigned values, reducing need for explicit type annotations.

Question 5

When should you use ‘readonly’ properties?

Answer 5

‘readonly’ is used when a property should be assigned once and never mutated, enforcing immutability.

Question 6

What is the difference between ‘public’, ‘protected’, and ‘private’ in TypeScript classes?

Answer 6

‘public’ is accessible everywhere, ‘protected’ is accessible in class and subclasses, ‘private’ is only within the class.

Question 7

What are ‘Type Guards’ in TypeScript?

Answer 7

Runtime checks (like ‘typeof’, ‘instanceof’, or custom predicates) that narrow down types within a block.

Question 8

What are ‘Generics’ in TypeScript and why are they useful?

Answer 8

Generics allow functions, classes, and interfaces to work with multiple types without losing type safety.

Question 9

What’s the difference between ‘any’, ‘unknown’, and ‘never’?

Answer 9

‘any’ disables type checking, ‘unknown’ is safer and requires type guards, ‘never’ is for unreachable code or exhaustive checks.

Question 10

How does ‘as const’ affect a variable?

Answer 10

It narrows down the type to a literal and marks properties as readonly, useful for immutable values.

Question 11

How would you implement Dependency Inversion Principle using TypeScript interfaces?

Answer 11

Define interfaces for service abstractions and inject concrete implementations where needed, decoupling high-level modules from low-level details.

Question 12

Explain how to use ‘Mapped Types’ in TypeScript.

Answer 12

Mapped types use syntax like { [K in keyof T]: T[K] } to iterate and transform properties of another type.

Question 13

How can you enforce immutability deeply in an object structure?

Answer 13

By using Recursive Utility Types or libraries like ‘deep-freeze’, since ‘readonly’ only affects top-level properties.

Question 14

How would you create a ‘type-safe API response’ type in TypeScript?

Answer 14

Use generics like ApiResponse<T> { data: T; error?: string; } to type API responses explicitly.</T>

Question 15

What is ‘discriminated union’ and how does it help?

Answer 15

Unions of objects with a common literal property (discriminator) that simplifies type narrowing and exhaustive checks.

Question 16

How do you create a ‘conditional type’ and give an example?

Answer 16

Use ternary operator Syntax like T extends U ? X : Y allows type logic. Example: type IsString<T> = T extends string ? true : false;</T>

Question 17

Explain the difference between ‘interface merging’ and ‘type alias’.

Answer 17

Interfaces can be merged across declarations, type aliases can’t. Useful for extending modules or libraries.

Question 18

How would you implement a type-safe builder pattern in TypeScript?

Answer 18

Use chained method returns with narrowed types at each step, leveraging generics for state enforcement.

Question 19

What is ‘keyof typeof’ pattern used for?

Answer 19

‘keyof typeof’ is used to create union of keys from an object, ensuring type-safe access or mapping.

Question 20

How would you explain the benefits of TypeScript to a team hesitant about adopting it?

Answer 20

TypeScript reduces runtime bugs, improves developer productivity with autocompletion and refactors, and scales better for large codebases.