What does the following mapped type with key filtering produce? type FunctionProperties = { [K in keyof T as T[K] extends Function ? K : never]: T[K] };

Question

TypeScript Professional — Hard

What does the following mapped type with key filtering produce? type FunctionProperties = { [K in keyof T as T[K] extends Function ? K : never]: T[K] };

Accepted Answer

The mapped type FunctionProperties filters properties of type T to only include those with function values, remapping non-function properties to never. This creates a new type with only function properties.