Question 1

What is a mapped type?

Accepted Answer

{ [K in keyof T]: ... } transforms each property systematically. Powers Partial/Readonly/Pick. Can remap keys with as clauses.

Question 2

Key remapping with as — example use?

Accepted Answer

Filter/rename keys: { [K in keyof T as Exclude<K, 'x'>]: T[K] }. Template literals can prefix keys (getX style). Extremely expressive.

Question 3

How do modifiers +, - work in mapped types?

Accepted Answer

Add/remove readonly or ? optionality on generated props. Example: { -readonly [K in keyof T]-?: T[K] } makes required mutable.

Question 4

Homomorphic mapped types — why special?

Accepted Answer

Preserve optional/readonly modifiers from the original when mapping keyof T. Important when wrapping existing object types.

Question 5

Build a DeepReadonly<T>?

Accepted Answer

Recursive mapped type making properties readonly including nested objects. Handle arrays/functions carefully. Watch recursion limits.

Question 6

Mapped types vs runtime mapping?

Accepted Answer

Mapped types erase at compile time — still need runtime loops for values. Don't confuse type transforms with JS Object.map.

Question 7

When mapped types hurt DX?

Accepted Answer

Excessive nesting yields opaque errors; provide named aliases and examples. Export intermediate types for consumers.

Mapped Types TypeScript Interview Questions

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