web3-listen-event.js

// both web3 and your contract have be defined
if(web3js && contract)
{
    const yourEvent = contract.YourEventName();
    yourEvent.watch(function(error, result) {

      if (!error) {
        
//         Callback return
//         Object - An event object as follows:

//         address: String, 32 Bytes - address from which this log originated.
//         args: Object - The arguments coming from the event.
//         blockHash: String, 32 Bytes - hash of the block where this log was in. null when its pending.
//         blockNumber: Number - the block number where this log was in. null when its pending.
//         logIndex: Number - integer of the log index position in the block.
//         event: String - The event name.
//         removed: bool - indicate if the transaction this event was created from was removed from the blockchain (due to orphaned block) or never get to it (due to rejected transaction).
//         transactionIndex: Number - integer of the transactions index position log was created from.
//         transactionHash: String, 32 Bytes - hash of the transactions this log was created from.
        
        // event arguments cointained in result.args object
        const { eventArg1, eventArg2, eventArg3 } = result.args;
        // new data have arrived. it is good idea to udpate data & UI
        addNewDataToStateAndUpdateUI();
      } else {
        // log error here
        console.log(error);
      }
    });