How to use the pusher-js/node function in pusher-js

const Bitstamp_WS = function(opts) {
    if (opts) {
      this.opts = opts
    } else {
      this.opts = {
        encrypted: true,
      }
    }

    this.client = new Pusher(BITSTAMP_PUSHER_KEY, {
      encrypted: this.opts.encrypted,
      // encrypted: true
    })

    // bitstamp publishes all data over just 2 channels
    // make sure we only subscribe to each channel once
    this.bound = {
      trade: false,
      data: false,
    }

    this.subscribe()
  }