Loop with await

Question

I want to run the following code:

const express = require('express');
const app = express();

app.post('/getTransaction', (req,res) => {
  let transaction = getTransaction(req.body.id);

  let logs = getEncodedLogs(transaction.logs);
  console.log('SEND'); // Gets called before getEncodedLogs() returns Array
  return res.send(Object.assign(transaction, { logs }));
});

async function getEncodedLogs(logs) {
  let logsDecoded = [];
  await Promise.all(logs.map(async (log) => {
    logsDecoded.push({
      logIndex: log.logIndex,
      data: log.data,
      topics: log.topics
    });
  }));
  console.log(logsDecoded);
  return logsDecoded;
}

app.listen(process.env.PORT || 8000);

The issue I'm having is that res.send(Object.assign(transaction, { logs })); gets run before getEncodedLogs() has returned the processed Array . I could rewrite it to let logs = await getEncodedLogs(transaction.logs); but that would block the process. Is there a better way?

Answer 1

getEncodedLogs(transaction.logs) doesn't return your logs, it returns a Promise that resolves to your logs when it has finished ¹ . The simplest way of getting this to work as expected would be to add a .then callback:

app.post('/getTransaction', (req, res) => {
    let transaction = getTransaction(req.body.id);

    getEncodedLogs(transaction.logs).then(logs => {
        console.log('SEND');
        res.send(Object.assign(transaction, { logs }));
    });
});

---

^{1. as all async functions do - the await syntax is just a convenient way of saying 'wait for this promise to finish before you carry on executing this function, please'}

Answer 2

So currently nothing in this example is actually async or needs to involve a Promise. getTransactions might but without seeing the implementation I can't tell. So therefore the answer could be represented like this

const express = require('express');
const app = express();

app.post('/getTransaction', (req,res) => {
  let transaction = getTransaction(req.body.id);

  let logs = getEncodedLogs(transaction.logs);
  console.log('SEND'); // Gets called before getEncodedLogs() returns Array
  return res.send(Object.assign(transaction, { logs }));
});

function getEncodedLogs(logs) {
  return logs.map((log) => {
    return {
      logIndex: log.logIndex,
      data: log.data,
      topics: log.topics
    };
  }));
}

app.listen(process.env.PORT || 8000);

If we dive deeper and look at the following part of the question

let logsDecoded = [];
await Promise.all(logs.map(async (log) => {
  logsDecoded.push({
    logIndex: log.logIndex,
    data: log.data,
    topics: log.topics
  });
}));

logDecoded is just a normal array and therefore the async function within map is actually not async.