How can I run an asynchronous function inside a loop and get a return value (Node.js/Javascript)

Question

I have a function containing a loop that iterates over an array of objects. On each iteration, I need to execute an asynchronous function and get a value back from the request however, I need each iteration of the for loop to pause and wait for the response before continuing.

For example:

const tokeniseCustomer = async () => {
const object [{"val1":1,"val2":2},{"val1":"a","val2":"b"}];
for (let index = 0; index < object.length; index += 1) {
const data = await getData(object[index]);
console.log(data);
//wait for data before progressing to next loop iteration
}
};

I thought by adding the "await" keyword in the above example, the loop would wait for the getData function to return a value.

The problem with the above code is that nothing gets logged to the console. I tried troubleshooting this by adding some extra logging. For example:

const tokeniseCustomer = async () => {
const object [{"val1":1,"val2":2},{"val1":"a","val2":"b"}];
for (let index = 0; index < object.length; index += 1) {
console.log('a')
const data = await getData(object[index]);
console.log('b');
console.log(data);
//wait for data before progressing to next loop iteration
}
};

The above code logs 'a' but not 'b'.

This is what my getData function looks like:

const getData = async (object) => {
  //get the data from MSSQL and return it
  const result = await MSSQLFUNCTION(object);
  return result;
};

I am really not sure what I am doing wrong. Am I incorrect in using for loops with await?

You don't show the entire code context, but the await getData() line won't actually await unless it's inside a function that uses the async keyword. I'm surprised you didn't get an error message that said as much. — kevintechie, Jan 08 '22 at 08:39
This might help: https://stackoverflow.com/a/37576787/17175441 Also the answers to this: [In JavaScript, does using await inside a loop block the loop?](https://stackoverflow.com/questions/44410119/in-javascript-does-using-await-inside-a-loop-block-the-loop/44410481) — aerial, Jan 08 '22 at 08:43

score 0 · Answer 1 · answered Jan 08 '22 at 08:59

Await can only be used inside an async function. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await#:~:text=It%20can%20only%20be%20used%20inside%C2%A0an%20async%20function

function sleep(ms) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}
const object = [
  { val1: 1, val2: 2 },
  { val1: "a", val2: "b" },
];
(async () => {
  for (let index = 0; index < object.length; index += 1) {
    console.log("a");
    const data = await sleep(5000)
    console.log("b");
    console.log(data);
  }
})();

Actually, the await is inside an async function. I'll update my code to show this. Thanks for the answer. — gunstonpaul, Jan 08 '22 at 09:46

Vignesh Murugan · Answer 2 · 2022-01-08T10:39:12.257

0

Can you try the snippet mentioned below: This basically runs a reducer over the array and waits for previous promise to complete before executing next one

const tokeniseCustomer = async () => {
  const object = [{"val1":1,"val2":2},{"val1":"a","val2":"b"}];
  object.reduce(async (acc, curr) => {
    await acc;
    return getData(curr);
  }, Promise.resolve(null))
 }

edited Jan 08 '22 at 10:39

answered Jan 08 '22 at 10:26

Vignesh Murugan

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How can I run an asynchronous function inside a loop and get a return value (Node.js/Javascript)

2 Answers2