@reactivex/ix-esnext-esm/iterable/findindex.d.ts

export declare function findIndex<T, S extends T>(source: Iterable<T>, predicate: (value: T, index: number) => value is S, thisArg?: any): number;
export declare function findIndex<T>(source: Iterable<T>, predicate: (value: T, index: number) => boolean, thisArg?: any): number;