Pattern for using Promise-based background jobs in Node + Express.

server.js

/* A quick reminder on how to use Promises with Node and Express in order to run potentially
 * time-consuming jobs asynchronously (assuming the jobs are able to run in the background thanks to
 * libuv and actually return Promises).
 *
 * Start the server, navigate to /startjob, then refresh a few times, the job should be in progress.
 * You can navigate elsewhere in the "app" in other browser tabs in the meanwhile (/). After about
 * 20s, you should get a result by refreshing the tab where you originally submitted the job.
 *
 * I hope this pattern will be useful e.g. for processing images with the `sharp` library (see
 * <http://sharp.dimens.io>).
 */
const express = require("/usr/lib/node_modules/express");
const app = express();
const PORT = process.env.PORT || 3000;
// this is where we'll store the results of our jobs by uuid once they're done
const JOBS = {};

app.get("/", (req, res) => {
  res.send("It works!");
});

app.get("/startjob", (req, res) => {
  let times = [100, 1000, 10000, 20000];
  let promises = [];
  for (let time of times) {
    promises.push(new Promise((resolve, reject) => {
      setTimeout(resolve, time, `${time} is done.`);
    }));
  }
  // obviously, you'd want to generate a real uuid here to avoid collisions
  let uuid = "uuid";
  Promise.all(promises).then(values => { JOBS[uuid] = values; });
  res.redirect(`progress/${uuid}`);
});

app.get("/progress/:uuid", (req, res) => {
  if (JOBS[req.params.uuid] === undefined) {
    res.send("Still processing your request.");
  } else {
    res.send(`Here's your result: ${JOBS[req.params.uuid]}.`);
    // instead of immediately deleting the result of the job (and making it impossible for the user
    // to fetch it a second time if they e.g. accidentally cancel the download), it would be better
    // to run a periodic cleanup task on `JOBS`
    delete JOBS[req.params.uuid];
  }
});

app.listen(PORT, () => {
  console.log(`Listening on localhost:${PORT}.`);
});