Typescript generic filtering of object keys by prefix/return

typings.ts

// FilterPrefixedKeys<{ a1, b1 }, 'a'> == { a }
export type FilterPrefixedKeys<T, S extends string> = T extends `${S}${infer _X}` ? T : never

// https://stackoverflow.com/questions/69038001/remove-string-from-template-literal-types-in-typescript
// GetPostPrefixSubstring<{ a1, b}, 'a'> == { 1 }
export type GetPostPrefixSubstring<T, S extends string> = T extends `${S}${infer X}` ? X : never