How to use the vary.append function in vary

function vary (reply, field) {
  var value = reply.getHeader('Vary') || ''
  var header = Array.isArray(value)
    ? value.join(', ')
    : String(value)

  // set new header
  if ((value = append(header, field))) {
    reply.header('Vary', value)
  }
}

if (options.credentials === true) {
        set('Access-Control-Allow-Credentials', 'true');
      }

      if (options.exposeHeaders) {
        set('Access-Control-Expose-Headers', options.exposeHeaders);
      }

      if (!options.keepHeadersOnError) {
        return await next();
      }
      try {
        return await next();
      } catch (err) {
        const errHeadersSet = err.headers || {};
        const varyWithOrigin = vary.append(errHeadersSet.vary || errHeadersSet.Vary || '', 'Origin');
        delete errHeadersSet.Vary;

        err.headers = Object.assign({}, errHeadersSet, headersSet, { vary: varyWithOrigin });

        throw err;
      }
    } else {
      // Preflight Request

      // If there is no Access-Control-Request-Method header or if parsing failed,
      // do not set any additional headers and terminate this set of steps.
      // The request is outside the scope of this specification.
      if (!ctx.get('Access-Control-Request-Method')) {
        // this not preflight request, ignore it
        return await next();
      }