I am trying to make a Singleton class to save some variable and use it from other class. But I am getting following error when I am trying to access it.

TypeError: Singleton.createInstance is not a function
at /Users/pankajsachdeva/Desktop/Pankaj/My Code/SmilaApp/SmilaApi/app.js:17:35
at Layer.handle [as handle_request] (/Users/pankajsachdeva/node_modules/express/lib/router/layer.js:95:5)
at next (/Users/pankajsachdeva/node_modules/express/lib/router/route.js:137:13)
at Route.dispatch (/Users/pankajsachdeva/node_modules/express/lib/router/route.js:112:3)
at Layer.handle [as handle_request] (/Users/pankajsachdeva/node_modules/express/lib/router/layer.js:95:5)
at /Users/pankajsachdeva/node_modules/express/lib/router/index.js:281:22
at Function.process_params (/Users/pankajsachdeva/node_modules/express/lib/router/index.js:341:12)
at next (/Users/pankajsachdeva/node_modules/express/lib/router/index.js:275:10)
at expressInit (/Users/pankajsachdeva/node_modules/express/lib/middleware/init.js:40:5)
at Layer.handle [as handle_request] (/Users/pankajsachdeva/node_modules/express/lib/router/layer.js:95:5)

Following is my Singelton class:

var mySingleton = (function() {
    let instance;
    let message;
  
    function createInstance() {
      if (!instance) instance = { setMessage, getMessage };
      return instance;
    }
    function setMessage(newMessage) {
      message = newMessage;
    }
    function getMessage() {
      return message;
    }
  
    return { createInstance }
  })();
  

I am trying to use it in following class:

    const express = require('express')
const Singleton = require('./mySingleton');

const app = express()
const port = 3000

app.post('/readyforunlock', (req, res) => {
    var firstInstance = Singleton.createInstance();
    firstInstance.setMessage("Message");
    console.log(firstInstance.getMessage());
    res.send(firstInstance.getMessage());
})


app.listen(port, () => {
  console.log(`Example app listening on port ${port}`)
})

I am very new to Node.js and not able to figure out why is it not working. I am trying to set some variable in one api and access it from another api. I am trying to achieve this using Singleton. Also please let me know if this correct approach or there are better options.

Regards