Accessing constructors variables in node. js

I am learning node.js

I have a class like this ->

 const client = require('prom-client');

class PrometheusController {
    constructor () {
        let counter = new client.Counter({ name: 'http_total_requests', namespace:"test", help: 'test' });
}

    get (fn) {
        this.counter.inc(); // Inc with 1
}

Node js complains that the counter is undefined.

I tried saving the this variable as the posts here recommend but that is not accessible either - javascript class variable scope in callback function

How do I access the constructors variables?