solidity – How to Listen to Smart Contract Events from a NodeJS-ExpressJS Server

A contract cannot listen to events of another contract. From Solidity docs:

Log and event data is not accessible from within contracts (not even from the contract that created a log).

web3.js is a wrapper around JSON-RPC, so another way of accessing event data is via "filters" in JSON-RPC such as eth_newFilter.

Note the dichotomy that a contract can't access events and web3.js is needed, but web3.js can't access return values from a contract invocation. So a pattern of using both an event and a return value like this may be needed:

event FooEvent(uint256 n);
function foo() returns (uint256) {
  FooEvent(1337);
  return 1337;
}

You can do this as following,

var event = contract.myEvent();
event.watch((err, res) => {
    console.log(res); // event response
    // Do something on event;
    event.stopWatching() // Stop watching for the event once you've done what you wanted to do.
});

It would btw be pretty nice if you could also post your node.js code.

EDIT: Whats important to know is that if you are connected to infura (test or mainnet) you need to connect to their websocket provider and now their http provider, so instead of for example: https://mainnet.infura.io/ you have to insert wss://mainnet.infura.io/wsas provider.