@buka/class-transformer-extra/dist/esm/src/array/flatten.js

class-transformer-extra contains methods that's aren't included in the class-transform package.

import { Transform } from 'class-transformer';
import { flatten } from 'ramda';
export function Flatten(options) {
    return Transform(function FlattenTransform({ value }) {
        return (Array.isArray(value) ? flatten(value) : value);
    }, options);
}