NodeJS TypeError: Cannot read property of undefined

var bodyParser = require('body-parser');

app.use(bodyParser.urlencoded({ extended: false }));

let obj = {
    o1: {
        a: 'abc'
    }
};

obj.o1     // gives {a: 'abc'}, as o1 is property obj which is an object.
obj.o1.a   // gives 'abc', as a is property of o1, which is property of obj.
obj.o2     // undefined, as there's no o2 property in obj.
obj.o2.a   // TypeError as there's no o2 property of obj and thus accessing property a of undefined gives error. 

module.exports = {
  async update(req, res) {
    const currency = await Currency.findByIdAndUpdate(req.params.id, req.body, {
      new: true
    });
    return res.json(currency);
  }
};

module.exports = {

  async getCurrency(id, params) {
      const currency = await Currency.findByIdAndUpdate(id, params, {
        new: true
      });
      return currency;
  }

  async update(req, res) {
    const currency = await getCurrency(req.params.id, req.body);
    return res.json(currency);
  }
};

BOT_KEY='something' node <filename>
// or 
export the BOT_KEY 
// or 
use `dotenv` and set the config in the config file.

const bot = bb({
  ....
  key: process.env.BOT_KEY || "some secret key",
  ....
});