@reactivex/ix-esnext-esm/asynciterable/first.d.ts

export declare function first<T, S extends T>(source: AsyncIterable<T>, predicate: (value: T, index: number) => value is S): Promise<S | undefined>;
export declare function first<T>(source: AsyncIterable<T>, predicate?: (value: T, index: number) => boolean | Promise<boolean>): Promise<T | undefined>;