Disclosure
Most of the problems under my TypeHero Challenges folder were either obtained from typehero.dev or from type-challenges repo. Purpose of these articles are just to document my approaches for my easy reference. Please visit the respective links for more info.
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Problem Description
Implement the JavaScript Array.includes function in the type system. A type takes the two arguments. The output should be a boolean true or false.
For example:
type isPillarMen = Includes<['Kars', 'Esidisi', 'Wamuu', 'Santana'], 'Dio'> // expected to be `false`Solutions
Approach: Using custom type equality check
type MyEqual<A, B> = (<T>() => T extends A ? 1 : 2) extends
(<T>() => T extends B ? 1 : 2) ? true : false
type Includes<T extends readonly any[], U> = T extends [infer First, ...infer Rest]
? MyEqual<First, U> extends true
? true
: Includes<Rest, U>
: false;