Why can I not return an array of objects in an async function?

Question

In node.js I am trying to get a list of bid and ask prices from a trade exchange website (within an async function). Within the foreach statement I can console.info() the object data(on each iteration) but when I put all of this into an array and then return it to another function it passes as 'undefined'.

const symbolPrice = async() => {
    let symbolObj = {}
    let symbolList = []

  await bookTickers((error, ticker) => {
    ticker.forEach(symbol => {
      if (symbol.symbol.toUpperCase().startsWith(starts.toUpperCase())) {

        symbolObj = {
          symbol: symbol.symbol,
          bid: symbol.bidPrice,
          ask: symbol.askPrice
        }
        console.info(symbolObj);

      }

      symbolList.push(symbolObj)


    });

    const results = Promise.all(symbolList)
    return results;

  });

}

const symbolPriceTest = async() => {

  const res = await symbolPrice(null, 'ETH', true);
  console.log(res)

}

I have tried pretty much everything I can find on the internet like different awaits/Promise.all()'s. I do admit I am not as familiar with async coding as I would like to be.

There are lots of problems here and you don't show how bookTickers() works or what you're trying to do with the result you generate so we can't really offer a complete solution. For starters, declare all your variables in the proper scope. Don't reuse variable names in a loop. Only use Promise.all() with an array of promises. Only use await when you are awaiting a promise. Don't mix promises and plain callbacks. — jfriend00
– jfriend00, Commented Nov 1, 2022 at 23:03
bookTickers() retrieves a current symbol bid and ask price through a WebSocket. With ticker being an array of objects(of the symbols and their bid/ask price). — David Williams
– David Williams, Commented Nov 1, 2022 at 23:10
Yo, hey you don't have to go off like that man listen I believe I have explained to the best of my ability what I am doing and what I am trying to achieve. bookTickers returns a promise or undefined. — David Williams
– David Williams, Commented Nov 1, 2022 at 23:22
Also I just want to add all of that data to an array and pass it on. Lets say to console.log() it to test functionality. — David Williams
– David Williams, Commented Nov 1, 2022 at 23:24

jfriend00 · Accepted Answer · 2022-11-01 23:43:36Z

2

So, if the basic problem here is to call bookTickers(), retrieve the asynchronous result from that call, process it and then get that processed result as the resolved value from calling symbolPrice(), then you can do that like this:

const { promisify } = require('util');
const bookTickersP = promisify(bookTickers);

async function symbolPrice(/* declare arguments here */) {
    let symbolList = [];

    const ticker = await bookTickersP(/* fill in arguments here */);
    for (let symbol of ticker) {
        if (symbol.symbol.toUpperCase().startsWith(starts.toUpperCase())) {
            symbolList.push({
                symbol: symbol.symbol,
                bid: symbol.bidPrice,
                ask: symbol.askPrice
            });
        }
    }
    return symbolList;
}

async function symbolPriceTest() {

  const res = await symbolPrice(null, 'ETH', true);
  console.log(res)

}

Things to learn from your original attempt:

Only use await when you are awaiting a promise.
Only use Promise.all() when you are passing it an array of promises (or an array of a mixture of values and promises).
Don't mix plain callback asynchronous functions with promises. If you don't have a promise-returning version of your asynchronous function, then promisify it so you do (as shown in my code above with bookTickersP().
Do not guess with async and await and just throw it somewhere hoping it will do something useful. You MUST know that you're awaiting a promise that is connected to the result you're after.
Don't reuse variables in a loop.
Your original implementation of symbolPrice() had no return value at the top level of the function (the only return value was inside a callback so that just returns from the callback, not from the main function). That's why symbolPrice() didn't return anything. Now, because you were using an asynchronous callback, you couldn't actually directly return the results anyway so other things had to be redone.

edited Nov 1, 2022 at 23:43

answered Nov 1, 2022 at 23:20

jfriend00

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6 Comments

David Williams Over a year ago

Ok I am going to be 100% honest I thought you just came in to berate me with no real teaching objective. Well played... thank you for your help and actually making this a teaching experience.

jfriend00 Over a year ago

@DavidWilliams - Sometimes it's just really hard to understand the questions here and those of us trying to answer get a bit frustrated when we can't see enough info in the question to understand what you're trying to do. I was just trying to get to the bottom of what you were trying to accomplish and how bookTickers() works. I am sometimes very direct in my approach which isn't always perceived the way I want, but isn't meant to do anything more than try to get to the bottom of things. Hope this answer helps.

danh Over a year ago

Seems to me the original comment was blunt but not unkind. I think I'd prefer a slightly different solution, though; one where the promisified bookTickersP is improved to return an array of { symbol, bid, ask }, and the symbol list is created with a filter.

jfriend00 Over a year ago

@danh - You're welcome to offer your own solution if you want to show something different. I prefer to keep the promisifed versions true to the original API so bookTickerP() here resolves to an array of ticker symbol objects which is the same data that bookTicker() calls its callback with. .filter() would certainly be appropriate - I just chose to not change that part of the logic.

danh Over a year ago

I like the pure promise version, too. Just suggesting a layer in between that reshapes the data, separate from a layer that uses it.

|

danh · Accepted Answer · 2022-11-02 00:36:13Z

Just a few thoughts on organization that might be reused in other contexts...

Promisify book tickers (with a library, or pure js using the following pattern). This is just the api made modern:

async function bookTickersP() {
  return new Promise((resolve, reject) => {
    bookTickers((error, ticker) => {
      error ? reject(error) : resolve(ticker);
    })
  });
}

Use that to shape data in the way that the app needs. This is your app's async model getter:

// resolves to [{ symbol, bid, ask }, ...]
async function getSymbols() {
  const result = await bookTickersP();
  return result.map(({ symbol, bidPrice, askPrice }) => {
    return { symbol: symbol.toUpperCase(), bid: bidPrice, ask: askPrice }
  });
}

Write business logic that does things with the model, like ask about particular symbols:

async function symbolPriceTest(search) {
  const prefix = search.toUpperCase();
  const symbols = await getSymbols();
  return symbols.filter(s => s.symbol.startsWith(prefix));
}

symbolPriceTest('ETH').then(console.log);

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