queryString option always prepends "?" even when result is empty

[fooddilsn]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

Use case

I'm building a proxy that strips certain authentication-related query params (e.g. token, workspaceId) before forwarding to the upstream. When those are the only params, the result is an empty query string, and the upstream receives a trailing ?.

app.get('/proxy/*', (request, reply) => {
  return reply.from('http://upstream/path', {
    queryString: (search, reqUrl) => {
      const params = new URLSearchParams(search);
      params.delete('token');
      params.delete('workspaceId');
      return params.toString();
    },
  });
});

Problem

The getQueryString function always prepends ? to the result of the queryString option, both for the function form and the object form, even when the result is an empty string:

function getQueryString (search, reqUrl, opts, request) {
  if (typeof opts.queryString === 'function') {
    return '?' + opts.queryString(search, reqUrl, request)
  }

  if (opts.queryString) {
    return '?' + querystring.stringify(opts.queryString)
  }

  if (search.length > 0) {
    return search
  }

  const queryIndex = reqUrl.indexOf('?')

  if (queryIndex > 0) {
    return reqUrl.slice(queryIndex)
  }

  return ''
}

This means there's no way to signal "no query string". Returning '' from the function (or passing an empty object {}) produces a trailing ? on the upstream request path (e.g., a request to /proxy/items?token=secret results in the upstream receiving /path? instead of /path).

By contrast, the default code path (no queryString option) correctly returns '' when there's no query string to forward.

Expected behavior

When the queryString result is an empty/falsy string, no ? should be appended, matching the behavior of the default code path.

Suggested fix

if (typeof opts.queryString === 'function') {
  const qs = opts.queryString(search, reqUrl, request)
  return qs ? '?' + qs : ''
}

if (opts.queryString) {
  const qs = querystring.stringify(opts.queryString)
  return qs ? '?' + qs : ''
}

Environment

• @fastify/reply-from: 12.6.0
• Node.js: 24.x

[fox1t]

Thanks for reporting. Could you open a PR?