How to subscribe to transaction with a smart contract in web3js?

Question

I am trying to write a web3js code. I have already written it using web3j (java) and now i want to do exactly same thing but using web3js. I have seen documentation and stuff, but i am not able to subscribe to events in web3js.

Below is my code written using web3j :-

        web3j =  Web3j.build(webSocketService);
        Web3ClientVersion web3ClientVersion = web3j.web3ClientVersion().send();
        ClientTransactionManager transactionManager = new ClientTransactionManager(web3j, RTKContractAddress);

        EthFilter RTKContractFilter = new EthFilter(DefaultBlockParameterName.LATEST, DefaultBlockParameterName.LATEST, RTKContractAddress);
        
        disposable = web3j.ethLogFlowable(RTKContractFilter).subscribe(log -> {

                String hash = log.getTransactionHash(); // Here we obtain transaction hash of transaction from the log that we get from subscribe
                String prevHash = flowableTransactionLog.get(numberOfTransactionsFetched.get() - 1).getTransactionHash();

                if(!hash.equals(prevHash)) {
                    flowableTransactionLog.add(log);
                    System.out.println("Chat ID : " + chat_id + " - Trx :  " + log.getTransactionHash());
                    Optional<Transaction> trx = web3j.ethGetTransactionByHash(hash).send().getTransaction();
                    trx.ifPresent(transaction -> allRTKTransactions.add(transaction));
                    numberOfTransactionsFetched.getAndIncrement();
                }
        }, throwable -> {
            throwable.printStackTrace();
            webSocketService.close();
            webSocketService.connect();
        });

In the above code, whenever there is a new trx with the smart contract, my code gets executed, and in that code, i add the received LOG in a ArrayList. Later, i read them when it becomes necessary.

So, in java subscribing to a smart contract was super easy. All i needed was the contract address. Using the contract address, i made a filter and using the filter, i subscribe to all the logs that will be made after the Latest block. Using the logs, I also fetched the transaction details.

But i am not able to do the same in web3js. One things i saw is that, to make a filter in web3js, i need to have the ABI of the contract, but the contract which i want to subscribe to is not open source and the ABI of the contract is not available online.

This is that token :- CRTS TOMO Token

What is the equivalent web3js code? Three problems/questions that i am facing are :-

1. I don't have contact ABI
2. Without ABI, how to make a filter
3. How to subscribe to the smart contract.

Answer 1

So i found the answer.

This is the equivalent code (Node.js) :-

const Web3 = require('Web3');
let web3 = new Web3(new Web3.providers.WebsocketProvider("wss://ws.tomochain.com"));

var subscription = web3.eth.subscribe('logs', {
    address: "0x2c78958660E734701f5D710CF5e967A0C8585cE5"
}, function(error, result){
    if (!error) {
        // Here goes the equivalent code for operation to be performed
        // when we get the JSON of new transactions. We can get the hash from the
        // JSON and then get the transaction from the hash and do same thing as i did 
        // in the java code.

        // If I just write the below line, then the respective output is given below the code.
        console.log(result);
    }
});

When new transactions are made with the contract, this type of output if given :-

{
  address: '0x2c78958660E734701f5D710CF5e967A0C8585cE5',
  topics: [
    '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef',
    '0x00000000000000000000000050cd10c496a20e98c953e88d6ba7b3f6bc21c589',
    '0x00000000000000000000000050cd10c496a20e98c953e88d6ba7b3f6bc21c589'

  ],
  data: '0x0000000000000000000000000000000000000000000000000de0b6b3a7640000',
  blockNumber: 25679866,
  transactionHash: '0x3f9d89a07560e59a82d6189d6714c933130f3df2a08f09e90249bf908930c109',
  transactionIndex: 2,
  blockHash: '0x5c605a409ec4e6c7ba28cf411b7ab4e17884cea9bbfb2ad632122ce16020f00d',
  logIndex: 2,
  removed: false,
  id: 'log_dd9d4710'
}