PickByType

In this medium-level challenge, you'll build a PickByType utility that selects only the properties from an object type T whose values are assignable to a given type U.

type OnlyBoolean = PickByType<{
name: string
count: number
isReadonly: boolean
isEnable: boolean
}, boolean> // { isReadonly: boolean; isEnable: boolean; }

Detailed Explanation

type PickByType<T, U> = {
  [K in keyof T as T[K] extends U ? K : never]: T[K]
}

How it works:

• We use a mapped type to iterate over all keys K in T.
• The as clause performs key remapping: for each key, we check whether T[K] extends U (whether the property's value type is assignable to U).
• If the condition is true, we keep the key K in the output type. If false, we remap it to never, which removes it entirely.
• The value type T[K] is preserved as-is for all keys that survive the filter.

This is the inverse of OmitByType, which removes properties matching the given type. Together, these two utilities demonstrate how key remapping with as enables powerful property filtering patterns.

This challenge helps you understand key remapping in mapped types and how to apply conditional property selection based on value types in real-world scenarios.