I can not run QZ-Tray if it inside a function on Express

Question

I try to make a web app to print on LQ-310 Printer using express and QZ-Tray. However, I cannot execute the printing process as expected, it only triggered to print if users hit the endpoint, if I put the QZ-Tray functions inside another function. I have tried to create a dedicated file for it and execute it using bash/shell script if users trigger the endpoint like this:

const printInvoice = (req, res) => {
    fs.writeFile(`src/invoice/invoice.json`, JSON.stringify(req.body), (err) => {
        if (err) throw err;
    });

    // shell script to run qz-tray
    const shell = require('shelljs');
    shell.echo('Running qz-tray');
    shell.exec('bash src/invoice/qz-tray.sh');

    fs.readFile(`src/invoice/invoice.json`, (err, data) => {
        if (err) throw err;
        res.json(JSON.parse(data));
    });
}

However, my terminal return no result, not even error message.

I also have set out to just do it inside one function on route, but found no result.

Note: this is my repo for the problem, Rusydy/print-qz

Answer 1

QZ Tray is most commonly a client-side library. This means that the JavaScript calls must originate from the client (browser), not the server (node).

Here's a very simple boilerplate layout.jade to get Express + Jade talking to QZ Tray. It's built on top of the express application generator.

• Note 1: Please excuse any fundamental app design or templating mistakes below as I don't have experience with Express -- or Jade for that matter.

• Note 2: Node/server/backend communication with QZ Tray is supported too, but this combination is much less common.

# put qz-tray.js in an accessible, public directory
curl -o public/javascripts/qz-tray.js https://raw.githubusercontent.com/qzind/tray/master/js/qz-tray.js

doctype html
html
  head
    title= title
    link(rel='stylesheet', href='/stylesheets/style.css')
    /***************************
     * code changes start here *
     ***************************/ 
    script(src='/javascripts/qz-tray.js', type='text/javascript')
    script.
      qz.websocket.connect().then(() => {
        return qz.printers.find()
      }).then(printers => {
        document.getElementsByTagName("p")[0].innerHTML += "<p>Found printers:</p><ul>";
        printers.forEach(printer => {
          document.getElementsByTagName("p")[0].innerHTML += "<li>" + printer + "</li>";
        });
        document.getElementsByTagName("p")[0].innerHTML += "</ul>";
      });
    /***************************
     *   end of code changes   *
     ***************************/ 
  body
    block content