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?

Answer 1

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); } })();

Answer 2

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))
 }