Typed String.prototype.includes

stringIncludes.ts

type FilterUnion<T extends string, Keys extends string> = {
  [Key in Keys extends infer R ? (R extends `${string}${T}${string}` ? R : never) : never]: string;
} extends infer O
  ? keyof O
  : never;

export const stringIncludes = <T extends string>(text: T) => <S extends string>(
  str: S
): str is FilterUnion<T, S> => str.includes(text);