AWS Lambda nodejs - problem with return values from await async function

Question

This is AWS Lambda function awaiting on async function. I can not get returned value. I need to do fetch data in loop as long as there are still some values on the server to be retrieved:

let do_search = true;
while (do_search) {
        const url = 'xxx';
        await fetchData(url)
            .then(response => {
                console.log("fetchData result:", response);
                if (response) {
                    console.log("Stop searching, no more data");
                    do_search = false;
                }
            })
.....

while my fetchData returns false value when there is still more data to be processed. fetchData function:

async function fetchData(url) {
...
console.log("Returning false");
return false;

The problem is even my fetchData returns false, my log are always:

Returning false
fetchData result: true

I have also tried to use other approach with:

 const createModelBInstance = async () => {
            let response = await fetchData(url)
            console.log("fetchData result:", response);
            if (response){
                do_search=false;
            }
           }

           await createModelBInstance();

Based on some examples on this forum. But exactly same problem, my fetchData returns false while "fetchData result: true".

Any working example ? That Promise values returned are causing simple code to be very complicated :(

Where does fetchData come from? Is that your code or an imported package? — jarmod
– jarmod, Commented Nov 11, 2019 at 21:57
Looking at your code if the response is equal to true than do_search is going to be false. Is that right? According to what you say it should be the opposite if response is equal to false than do_search is false. In that case you might want to do this if(!response) ... — Matteo
– Matteo, Commented Nov 12, 2019 at 1:17
@Madeo - yes - correct, if my function returns true, do_search should be set to false and the loop should finish. The problem is that my function returns false but returned value is always true here. — user2913139
– user2913139, Commented Nov 12, 2019 at 10:35
@Jarmod, fetchData() is my own async function which just returns true (if we have no more data to read) or false (if we still have some data to be read). — user2913139
– user2913139, Commented Nov 12, 2019 at 10:36
If FetchData returns true or false then that’s a fulfilled promise. Also, you shouldn’t use both await and a then handler. Use one or the other. I also suspect your FetchData code needs to be reviewed and corrected. — jarmod
– jarmod, Commented Nov 12, 2019 at 12:37

jarmod · Accepted Answer · 2019-11-12 16:49:26Z

You shouldn't write hard loops in JavaScript. It's single-threaded.

Here's an example of a fetchData method that sleeps for 1 second, then indicates whether or not data is available (available with 10% likelihood, not available 90% likelihood). This is purely to simulate your asynchronous URL fetcher.

const getRandom = (low, count) => {
  return Math.floor((Math.random() * count) + low)
}

const fetchData = () => {
  const promise = new Promise((resolve, reject) => {
    setTimeout(() => {
      const rand = getRandom(1, 10);
      if (rand % 7) {
        resolve({ rand });
      } else {
        resolve({ data: 'Here is the data', rand });
      }
    }, 1000);
  });

  return promise;
}

Here's an example of calling this code on an interval, every 2 seconds, until data becomes available.

const interval = setInterval(() => {
  fetchData().then(rc => {
    console.log(rc);
    if (rc.data) {
      // do something with rc.data
      clearInterval(interval);
    }
  });
}, 2000);

An example of the output:

{ rand: 2 }
{ rand: 10 }
{ rand: 1 }
{ data: 'Here is the data', rand: 7 }

not sure what i was doing wrong, you are 100% right, all working fine now, thanks @Jarmod !

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