import { Transform, TransformOptions } from "stream";

export function flatMap<T, R>(
    mapper:
        | ((chunk: T, encoding: string) => R[])
        | ((chunk: T, encoding: string) => Promise<R[]>),
    options?: TransformOptions,
): Transform {
    return new Transform({
        ...options,
        async transform(chunk: T, encoding, callback) {
            (await mapper(chunk, encoding)).forEach(c => this.push(c));
            callback();
        },
    });
}