Express request no longer available on fileStream.on('close') - node.js

Question

I have an implementation of a POST router in node.js with express. The body of the post is a list of files, and the idea is that these files should be zipped and a link to the resulting zip file should be returned. I'm trying to use 'express-session' to store the progress in terms of number of files zipped as well as the file name of the zip upon completion. Updating the number of files zipped (req.session.current) works fine, but for some reason I can't set the file name (req.session.zipFile) in the session when the fileStream closes. I'm suspecting that the req object is no longer valid by the time fileStream.on('close') is reached, but I'm not sure how to handle that. I can't access the session data without a request.
This is the complete router implementation:
const express = require('express');
const router = express.Router();
const fs = require('fs');
const ZIP_FILES_PATH = require('../constants');
router.post('/', function(req, res, next){
const json = JSON.parse(req.body.data);
const path = json["base"];
if (!path || path.length === 0){
res.render("error", {msg: "JSON missing 'base'"})
return;
}
const files = json["files"];
if (!files || files.length === 0){
res.statusCode = 400;
res.send("JSON missing 'files'");
return;
}
if (!fs.existsSync(path)) {
res.statusCode = 400;
res.send("Directory '" + path + "' does not exist");
return;
}
try{
var sessData = req.session;
sessData.total = files.length ;
sessData.current = 0;
sessData.zipFile = '';
zipFiles(path, files, req, res);
}catch(error){
res.statusCode = 500;
res.send("The files could not be zipped");
return;
}
res.render("zipping")
});
module.exports = router;
const zipFiles = (path, files, req, res) => {
const zipFile = require('crypto').createHash('md5').update(path).digest("hex") + "_" + new Date().getTime() + ".zip";
const archiver = require('archiver');
const fileStream = fs.createWriteStream(ZIP_FILES_PATH + zipFile);
const archive = archiver('zip', {
gzip: true,
zlib: { level: 9 }
});
archive.on('error', function(err) {
throw err;
});
fileStream.on('close', function() {
req.session.zipFile = "/download?file=" + zipFile; //Doesn't stick!
});
archive.pipe(fileStream);
files.map(file => zipSingleFile(path, file, req, archive));
archive.finalize();
}
const zipSingleFile = (path, file, req, archive) => {
fs.existsSync(path + "/" + file) ? archive.file(path + "/" + file, { name: file }) : null;
req.session.current = req.session.current + 1;
}

Answer 1

zipFiles() is asynchronous. That means it doesn't block and finishes some time later. So, in the flow of your request handler, you call
res.render("zipping")
before zipFiles() finishes. And, res.render() will send the response and close the output stream.
If you want to wait to call res.render() until you're done with the zipFiles() call, then you need to either call it from within zipFiles() when it's all done or you need to add a callback to zipFiles() so it can communicate back when it's done.
Also, is there any reason you can't set req.session.zipFile = "/download?file=" + zipFile; before you call res.render()?
Also, depending upon how you have your session configured (which you don't show us any code for), just doing this:
req.session.zipFile = "/download?file=" + zipFile;
may not be enough to actually save that value into the session. You may also have to call .save() on the session.
And, you are setting:
req.session.zipFile
at the end of your zipping process so any requests that arrive from that session before your zipping finishes won't see that session variable set either (this seems like a concurrency problem to me).