How to return result as variable from provider.on?

[adityawarmanfw]

Hi, I want to listen to pending transactions using provider.on, and it works with this code:

const streamPendingTx = (connect) => {
    connect.provider.on("pending", (pendingHash) => {
        console.log(pendingHash);

    });
};

(async () => {
    streamPendingTx(connectWSS())
})()

Now I want to pass the result from the provider.on to a variable, but I can't figure out how to do that.

Here's the function I wrote:

const streamPendingTx = (connect) => {
    let tx;
    connect.provider.on("pending", (pendingHash) => {
        tx = pendingHash;
    })
    console.log(tx);
    return tx;
};

I don't really get why this isn't working. Can somebody help? Thanks.

[zemse]

You may use a Promise;

const streamPendingTx = (connect) => {
    return new Promise(resolve => {
        connect.provider.on("pending", (pendingHash) => {
            resolve(pendingHash);
        })
    })
};


(async () => {
    const result = await streamPendingTx(connectWSS())
    console.log(result);
})()

[adityawarmanfw]

@zemse Thanks! I figured out that I can also use a callback, though I'm not sure which one is the best practice?

const streamPendingTx = (connect, callback) => {
        connect.provider.on("pending", (pendingHash) => {
            callback(pendingHash);
        })
    })
};

(async () => {
    streamPendingTx(connect, (res) => {
    console.log(res);
})()

[zemse]

Yeah, callbacks have been the way to do things all this time. The catch with this is, what if you had to do some tasks serially?

doFirstJob((first) => {
  doSecondJob(first, (second) => {
    doThirdJob(second, (third) => {
      console.log(third);
    })
  })
})

Then Promises were introduced to prevent so many callbacks

doFirstJob()
.then(first => doSecondJob(first))
.then(second => doThirdJob(second))
.then(third => console.log(third))

Then the ES8 async-await

const first = await doFirstJob();
const second = await doSecondJob(first);
const third = await doThirdJob(second);

IMO the async-await is very readable

[adityawarmanfw]

@zemse Thanks again for the detailed reply. However, I run into another problem, can you help me? I don't really know if this is a bug or a normal behaviour. I've add it to issue #2019.

So when listening to pending transactions, anything inside the connect.provider.on("pending", (pendingHash) => { will be executed as many times as the "pending" data retrieved. I want to find certain transaction using this findingTx() that contains some filter on pendingTx.data:

connect.provider.on("pending", async (txHash) => {
        if (txHash) {
            let pendingTx = await connect.provider.getTransaction(txHash);
            if (pendingTx) {
                const found = await findingTx(pendingTx);
                console.log(found);
            }
       }
});

Yet the console.log(found) giving same log multiple times with the same value.
Is there a good way to prevent this? I can't use provider.once because it immediately stop after finding only one pending txHash.

I attach this code to simulate the pending event stream, and my workaround to make the code only output single time:

const events = require('events');

const eventEmitter = new events.EventEmitter();

const listening = (callback) => {
  let num = 0;
  eventEmitter.on('scream', (scream) => {
    let obj;
    if (scream)
      process.stdout.write(`${scream} \r`)
    if (scream == 'Jack') {
      obj = { no: num += 1, value: scream }
    }
    callback(obj)
  });
  for (let j = 0; j < 10; j++) {
    eventEmitter.emit('scream', 'Bob');
    eventEmitter.emit('scream', 'Jack');
    eventEmitter.emit('scream', 'Nina');
  }
};

const init = () => {
  listening(function (res) {
    if (res && res.no === 1) {
      process.stdout.write(`\n\n`)
      let values = res.value
      console.log(res)
      console.log("everything in this block will only be fired once")
    }
  });
}

init()

Is there any better way to this?