Interface vs type alias decision in Typescript - Quick Revision & Key Differences

Recall & Review

beginner

What is a key difference between an interface and a type alias in TypeScript?

An interface can be extended or implemented multiple times, allowing declaration merging, while a type alias cannot be reopened once declared.

beginner

Can a type alias represent union or intersection types in TypeScript?

Yes, type aliases can represent complex types like unions (|) and intersections (&), which interfaces cannot directly express.

intermediate

When should you prefer using an interface over a type alias?

Use interface when you want to take advantage of declaration merging or when defining object shapes that might be extended or implemented by classes.

beginner

Is it possible to use a type alias to define a function signature in TypeScript?

Yes, type aliases can define function types, for example: type Fn = (x: number) => string;.

intermediate

What happens if you declare two interfaces with the same name in TypeScript?

TypeScript merges the declarations, combining their members into a single interface.

Which of the following can be extended by other declarations in TypeScript?

A Both interface and type alias

B Type alias

C Interface

D Neither interface nor type alias

Only interface supports extension and declaration merging in TypeScript.

Which can represent a union type in TypeScript?

A Interface

B None

C Both

D Type alias

Type aliases can represent union types, but interfaces cannot.

If you want to define a function type, which is the best choice?

A Interface

B Type alias

C Either interface or type alias

D Neither

Type aliases are commonly used to define function types.

What happens when two interfaces with the same name are declared?

A Interfaces are merged

B The second interface overwrites the first

C Error: duplicate interface

D Only the first interface is used

TypeScript merges interfaces with the same name into one combined interface.

Which is NOT true about type aliases?

A They support declaration merging

B They cannot be reopened once declared

C They can define function types

D They can represent primitives, unions, intersections

Type aliases do NOT support declaration merging; only interfaces do.

Explain when you would choose an interface over a type alias in TypeScript.

Think about reusability and extension features.

Describe the capabilities of type aliases that interfaces do not have.

Focus on complex type expressions.