Question 1

What does Partial<T> do?

Accepted Answer

Makes all properties optional — useful for patches/updates. Shallow only. Deep partial needs custom mapped types.

Question 2

Pick vs Omit?

Accepted Answer

Pick selects keys; Omit removes keys from a type. Prefer them over manual duplication. Combine with Partial for update DTOs.

Question 3

Record<K, V> use cases?

Accepted Answer

Object type with keys K and values V — dictionaries and maps of known keys. Prefer Map at runtime when keys are dynamic/frequent add-delete. Record is a type.

Question 4

Required and Readonly?

Accepted Answer

Required removes optionality; Readonly makes props readonly. Shallow. Useful for freezing config types and completing option bags.

Question 5

ReturnType and Parameters?

Accepted Answer

Extract return type or parameter tuple from a function type. Great for wrapping APIs without importing private types. Doesn't run the function.

Question 6

NonNullable and Awaited?

Accepted Answer

NonNullable removes null|undefined; Awaited unwraps Promise-like chains. Common in async data pipelines and strict null cleanup.

Question 7

When should you write custom utilities instead of built-ins?

Accepted Answer

When you need deep variants, key remapping, or domain-specific transforms. Keep utilities documented and tested via type tests (expectType).

Utility Types TypeScript Interview Questions

Other TypeScript interview topics