Filter

In this medium-level challenge, you'll implement Filter<T, Predicate> which takes a tuple T and a primitive type (or union of primitive types) Predicate, and returns a new tuple containing only the elements that extend Predicate.

type Falsy = false | 0 | '' | null | undefined

type cases = [
  Expect<Equal<Filter<[0, 1, 2], 2>, [2]>>,
  Expect<Equal<Filter<[0, 1, 2], 0 | 1>, [0, 1]>>,
  Expect<Equal<Filter<[0, 1, 2], Falsy>, [0]>>,
]

Detailed Explanation

We recursively walk through the tuple, checking each element against the predicate and accumulating matches.

type Filter<T extends any[], P> = T extends [infer First, ...infer Rest]
  ? First extends P
    ? [First, ...Filter<Rest, P>]
    : Filter<Rest, P>
  : []

How it works:

• We use infer First and ...infer Rest to destructure the tuple into its head and tail
• First extends P checks if the current element matches the predicate type (including union members)
• If it matches, we include First in the result and recurse on Rest
• If it does not match, we skip it and recurse on Rest
• The base case returns an empty tuple when the input is exhausted

This challenge helps you understand recursive tuple filtering with conditional types and how to apply this concept in real-world scenarios.