als-send-file/lib/etag.js

file serving with advanced options for caching, headers, and error handling, compatible with Express middleware.

/**
 * Generates an ETag and handles If-None-Match and If-Modified-Since headers.
 * @param {http.IncomingMessage} req - The request object.
 * @param {http.ServerResponse} res - The response object.
 * @param {Object} stats - The file stats object.
 * @returns {boolean} - Returns true if the response was not modified, otherwise false.
*/

function generateETag(req, res, stats) {
   let { mtimeMs, mtime, ino } = stats
   const etag = ino.toString() + mtimeMs.toString();

   res.setHeader('Last-Modified', mtime.toUTCString());
   res.setHeader('ETag', etag);

   const clientEtag = req.headers['if-none-match'];
   const clientLastModified = req.headers['if-modified-since'];
   if (!clientEtag && !clientLastModified) return false;
   const lastModifiedTime = new Date(clientLastModified).getTime()
   let notModified = false
   if(!isNaN(lastModifiedTime)) {
      const roundedTimeMs = new Date(Math.floor(mtimeMs / 1000) * 1000); // round Last Three Digits To Zero
      notModified = lastModifiedTime >= roundedTimeMs
   }
   if (clientEtag === etag || notModified) {
      res.statusCode = 304;
      res.end();
      return true;
   }
   
   return false;
}

module.exports = generateETag;