How to loop req query in Nodejs, the code doesn't work and is giving me an undefined or Nan output

Here is my code:

const express = require('express');
const app = express();
const catalyst = require('zcatalyst-sdk-node');
app.use(express.json()); // for parsing

app.get('/', async (req, res) => {
    const catalystApp = catalyst.initialize(req);

    console.log(req.query);

    for (let i = 0; i < req.query.quantity; i++) {
        console.log(req.query.appliance_deck_ + `${i}`); // OUTPUT: undefined0
        console.log(req.query.appliance_deck_ + i); // OUTPUT: NAN
    }
    
});

module.exports=app;

Thank you.