Get all child terms from already retrieved list of parent terms

GetAllChildTerms.ts

// note: in a previous step "parentTerms" has already been retrieved; now we want all child terms down the hierarchy

const childTermsPromise = (parentTerms: ((ITermData & ITerm)[])): Promise<(ITermData & ITerm)[]> => {
    const termsWithChildTerms = [...parentTerms.filter(term => term.TermsCount > 0)];
    if (termsWithChildTerms.length == 0) {
        return Promise.resolve(parentTerms);
    }
    // get all child terms
    const childPromises = termsWithChildTerms.map(childTerms => childTerms.terms.get().then(values => childTermsPromise(values)));
    // wait for child terms and concat with parent terms
    return Promise.all(childPromises).then(result => result.reduce((prev, current) => prev.concat(current), [])).then(r => r.concat(parentTerms));
};

return childTermsPromise(alreadyRetrievedParentTerms);