Includes

Search arrays for exact type matches with Includes! 🔍

In this easy-level challenge, you'll implement the JavaScript Array.includes function in TypeScript's type system. This type checks whether a specific type exists within a tuple and returns a boolean literal type (true or false).

This challenge teaches important concepts about type equality, recursive type checking, and handling edge cases like readonly modifiers. You'll learn why TypeScript's extends isn't always sufficient for type comparison.

type isPillarMen = Includes<['Kars', 'Esidisi', 'Wamuu', 'Santana'], 'Dio'> // expected to be `false`

Detailed Explanation

The Includes type requires careful handling of type equality, as extends alone isn't sufficient for exact matching.

type Equal<X, Y> =
  (<T>() => T extends X ? 1 : 2) extends <T>() => T extends Y ? 1 : 2
    ? true
    : false
 
type Includes<T extends readonly any[], U> = T extends [
  infer First,
  ...infer Rest,
]
  ? Equal<First, U> extends true
    ? true
    : Includes<Rest, U>
  : false

Key concepts:

• Uses a helper Equal type for exact type comparison
• Recursively checks each element using pattern matching
• Equal ensures exact matches (e.g., 1 | 2 doesn't match 1)
• Handles edge cases like readonly modifiers and union types correctly

This pattern is essential for building robust type-level algorithms that require precise type matching.