Filter

Problem Link

Problem

Implement the type Filter<T, Predicate> which filters out elements from a tuple T that do not satisfy the Predicate type.

type Filtered = Filter<[1, 2, 3, 'a', 'b', 1], number> // [1, 2, 3, 1]

Solution

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:

1. Recursively destructure the tuple into First and Rest.
2. If First extends P, keep it in the output tuple.
3. Otherwise, skip it and continue with Rest.
4. Base case: empty tuple returns [].

Key Takeaways

• The standard “recursive tuple filter” pattern in TypeScript type-land mirrors Array.prototype.filter at the type level.
• First extends P is a subtype check — it keeps elements that are assignable to P.
• Building the result with [First, ...Filter<Rest, P>] preserves tuple structure and element types.