Type System & Custom Types

Question 1

What is an interface in TypeScript?

Answer 1

An interface defines a contract for object shapes, describing property names and their types.

Question 2

What is a type alias in TypeScript?

Answer 2

A type alias creates a new name for a type using the ‘type’ keyword.

Question 3

What is the difference between interface and type in TypeScript?

Answer 3

Interfaces are best for object shapes and can be extended or merged; types are more flexible and can represent primitives, unions, tuples, and more.

Question 4

When should you prefer an interface over a type?

Answer 4

Prefer interfaces when defining object shapes or class contracts, as they support declaration merging and extension.

Question 5

When should you prefer a type over an interface?

Answer 5

Prefer type when using unions, intersections, or working with primitives and complex type compositions.

Question 6

What is an enum in TypeScript?

Answer 6

An enum is a special TypeScript feature for defining named constants that can be numeric or string-based.

Question 7

What are the advantages of enums?

Answer 7

Enums improve code readability and reduce magic numbers or strings in the codebase.

Question 8

What are the disadvantages of enums?

Answer 8

Enums increase bundle size and add runtime code, unlike union literal types which are more lightweight.

Question 9

What is a literal type in TypeScript?

Answer 9

A literal type restricts a variable to have a specific value, like ‘on’ | ‘off’.

Question 10

What is a union type in TypeScript?

Answer 10

A union type allows a variable to hold one of several specified types, e.g., string | number.

Question 11

What is an intersection type in TypeScript?

Answer 11

An intersection type combines multiple types into one, requiring all properties of each to be present.

Question 12

What is a tuple in TypeScript?

Answer 12

A tuple is a fixed-length array with specified types for each position, e.g., [string, number].

Question 13

What is a use case for tuples?

Answer 13

Use tuples when working with pairs or structured but fixed-length heterogeneous data, like coordinates or key-value pairs.

Question 14

What is a potential gotcha with enums?

Answer 14

Unlike other types, enums exist at runtime, so they can unexpectedly increase the size of transpiled JavaScript.

Question 15

Can interfaces extend other interfaces?

Answer 15

Yes, interfaces can extend one or multiple other interfaces using the ‘extends’ keyword.

Question 16

What is a best practice for using union types?

Answer 16

Use union types for constrained input values or status flags to increase code safety.

Question 17

What is a real-world tradeoff between enums and literal unions?

Answer 17

Enums add runtime overhead, while literal types offer better tree-shaking and are more performant in front-end apps.

Question 18

What’s the impact of type aliases on system design?

Answer 18

They improve abstraction, reduce duplication, and make the codebase easier to maintain.

Question 19

What are the architectural implications of using interfaces extensively?

Answer 19

Interfaces can enforce contracts across layers and improve modularity and separation of concerns.

Question 20

What’s a performance benefit of literal union types over enums?

Answer 20

Literal types are erased during compilation and do not exist at runtime, reducing output size.

Question 21

What is a common interview question about interfaces vs types?

Answer 21

What are the differences between type aliases and interfaces in TypeScript and when would you use one over the other?

Question 22

What’s a potential issue when using complex intersection types?

Answer 22

Deeply nested or conflicting property types can make intersections hard to debug and maintain.

Question 23

How can union types help with fault tolerance?

Answer 23

Union types constrain input values, reducing invalid states and runtime errors.

Question 24

What is a debugging benefit of using literal types?

Answer 24

Literal types provide clearer compiler errors and auto-completions for restricted values.

Question 25

What is a potential gotcha when using tuples?

Answer 25

Accessing tuple elements out of bounds or misunderstanding their fixed structure can lead to bugs.

Question 26

What is declaration merging?

Answer 26

Declaration merging allows multiple declarations of the same interface to be combined automatically.

Question 27

What’s a best practice for extending types or interfaces?

Answer 27

Favor composition over duplication and use 'extends' for shared contracts to promote reuse.

Question 28

What’s a disadvantage of intersection types?

Answer 28

Complexity increases with deeply nested or overlapping properties, which can reduce readability.

Question 29

Can type aliases be recursive?

Answer 29

Yes, type aliases can reference themselves recursively for complex type structures like trees or JSON.

Question 30

What’s a common use case for string literal types?

Answer 30

Defining allowed values for input fields, configuration flags, or component props.

Question 31

What’s the syntax to define a type alias for a union of string values?

Answer 31

type Status = 'active' | 'inactive' | 'pending';

Question 32

What is the syntax to define a tuple with a string and number?

Answer 32

let pair: [string, number] = ['hello', 42];

Question 33

Can tuples have optional elements?

Answer 33

Yes, tuples can have optional elements using ? after the type: [string, number?]

Question 34

What’s the advantage of interfaces in large codebases?

Answer 34

They help enforce contracts and make it easier to reason about shapes of objects across modules.

Question 35

What’s the impact of using enums in server-side TypeScript apps?

Answer 35

Their runtime availability can be beneficial for reflection and validation but may increase bundle size.

Question 36

Can interfaces describe function types?

Answer 36

Yes, interfaces can be used to describe function signatures using call signatures.

Question 37

What’s the difference between const enums and regular enums?

Answer 37

const enums are inlined at compile time and do not generate runtime code.

Question 38

What’s a common interview question about union vs intersection types?

Answer 38

Explain the difference between union and intersection types with examples and when to use each.

Question 39

What is an example of an intersection type?

Answer 39

type A = { a: string }; type B = { b: number }; type AB = A & B;

Question 40

What’s a good way to organize type aliases and interfaces in a project?

Answer 40

Group related types in separate files or folders and use barrel exports for easy imports.

Question 41

What is the downside of excessive type composition?

Answer 41

Over-composing types can lead to poor readability and hard-to-debug compiler errors.

Question 42

What is the main benefit of using type aliases for primitives?

Answer 42

Improves code readability and semantic meaning, e.g., type UserID = string;

Question 43

What does TypeScript do with types at runtime?

Answer 43

Types are erased at compile time and do not exist at runtime; they are purely for static checking.

Question 44

What’s a best practice for naming types and interfaces?

Answer 44

Use PascalCase and descriptive names like UserResponse or IConfigOptions.

Question 45

What is a benefit of using tuples over arrays?

Answer 45

Tuples provide predictable positions and types, which increases type safety for fixed-structure data.

Question 46

How do literal types improve debugging?

Answer 46

They produce clear error messages when a value doesn't match one of the expected literals.

Question 47

Can interfaces be implemented by classes?

Answer 47

Yes, interfaces define contracts that classes can implement using the 'implements' keyword.