Exclude

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 built-in Exclude<T, U>

Exclude from T those types that are assignable to U

For example:

type Result = MyExclude<'a' | 'b' | 'c', 'a'> // 'b' | 'c'

Solutions

Approach 1

type MyExclude<T, U> = T extends U ? never : T

Suppose T = 'a' | 'b' | 'c' and U = 'a'. This works as if we’re looping through each item in T and comparing it to U:

1. Does 'a' extend 'a'? Skip.
2. Does 'b' extend 'a'? No, so keep.
3. Does 'c' extend 'a'? No, so keep.
4. Form a union of the kept values, i.e, 'b' | 'c'.