LogSubscription.js

const Web3 = require('web3');
const abi = require('./abi.json'); // ABI of Subscription.sol in "build" directory on truffle project.

// noinspection JSUnresolvedVariable
const web3 = new Web3(
  new Web3.providers.WebsocketProvider(config.contract.provider),
);

const subscriptionContract = new web3.eth.Contract(
  abi,
  '0x000000000000000000000000000000000000000', // Your Subscription.sol contract address.
);

const subscription = subscriptionContract.events
  .SubscriptionPurchase({ filter: {} }, (error, event) => {
    if (error) return console.error(error);
    console.log('Successfully subscribed!', event);
    // TODO add user's expiration to database.
  })
  .on('data', event => {
    console.log(event); // same results as the optional callback above
  })
  .on('changed', event => {
    // remove event from local database
  })
  .on('error', console.error);

// unsubscribes the subscription
subscription.unsubscribe((error) => {
  if (error) return console.error(error);

  console.log('Successfully unsubscribed!');
});