Public Type

In this medium-level challenge, you'll implement a PublicType utility that removes all properties whose keys start with an underscore _, exposing only the "public" members of an object type.

type cases = [
  Expect<Equal<PublicType<{ a: number }>, { a: number }>>,
  Expect<Equal<PublicType<{ _b: string | bigint }>, {}>>,
  Expect<Equal<PublicType<{ readonly c?: number }>, { readonly c?: number }>>,
  Expect<Equal<PublicType<{ d: string; _e: string }>, { d: string }>>,
  Expect<Equal<PublicType<{ _f: () => bigint[] }>, {}>>,
  Expect<Equal<PublicType<{ g: '_g' }>, { g: '_g' }>>,
  Expect<Equal<PublicType<{ __h: number; i: unknown }>, { i: unknown }>>,
]

Detailed Explanation

type PublicType<T extends object> = {
  [K in keyof T as K extends `_${string}` ? never : K]: T[K]
}

How it works:

• We use a mapped type to iterate over all keys K in T.
• The as clause performs key remapping using a template literal pattern: K extends `_${string}` checks if the key starts with an underscore.
• If the key starts with _, it is remapped to never, effectively removing it from the resulting type.
• All other keys are preserved along with their original value types, modifiers like readonly, and optionality.
• Note that a property whose value is a string starting with _ (like { g: '_g' }) is kept, since the filter applies to the key name, not the value.

This challenge helps you understand template literal types combined with key remapping, and how to apply this pattern for convention-based type filtering in real-world scenarios.