Question 1

What is declaration merging for interfaces?

Accepted Answer

Same-name interfaces combine members — useful for extending lib types. Unexpected merges can widen APIs. Prefer module augmentation intentionally.

Question 2

How do you extend interfaces?

Accepted Answer

extends clause for one or more bases; resulting type must be assignable consistently. Conflicts must be compatible. Classes can implement interfaces.

Question 3

Optional vs readonly properties?

Accepted Answer

?: may be absent; readonly prevents reassignment of the property. readonly is shallow. Optional undefined vs missing differs under exactOptionalPropertyTypes.

Question 4

Index signatures — uses and pitfalls?

Accepted Answer

Allow dynamic keys with a value type; can force all properties to be assignable to that type. Prefer maps/Records for dictionaries. String indexes interact with declared fields carefully.

Question 5

How do function types appear in interfaces?

Accepted Answer

Call signatures or property-function members; prefer method vs property for bivariance nuances. this typing in methods matters. Overload call signatures possible.

Question 6

Interface for API response modeling tips?

Accepted Answer

Model only what you use; validate at runtime; don't trust backend blindly. Separate DTO types from domain types when shapes diverge.

Question 7

When do interfaces become an anti-pattern?

Accepted Answer

God interfaces, premature abstraction, and over-modeling JSON 1:1 without validation. Prefer smaller contracts and composition. Codegen OpenAPI carefully.

Interfaces TypeScript Interview Questions

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