How to exit from node script after a asynchronous operations completed?

Question

I am trying to run a loop and performing async operation in it. I am trying to delete documents from Mongodb. I want the script to terminate when all delete operations completed. I tried using process.exit(0), but when I use this method to terminate the script, no number is getting deleted .

async function deletor() {
mobile_numbers.forEach(async (mobile_number) => {
    await db.collection(db_config.collectionName).remove(
    { mobile: mobile_number }
    )
})
}
deletor()
process.exit(0)

When all delete operations completed, then the script should terminate. but it terminates before the delete operation completes. I want to add this script to cron job, and I want to terminate the script when all the async operations are completed.

How can this be done?

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– T.J. Crowder, Commented Dec 29, 2018 at 17:08
Node will do so automatically unless you do something to prevent it. What specific problem are you having? — T.J. Crowder
– T.J. Crowder, Commented Dec 29, 2018 at 17:08
I have modified the question, I am trying multiple async operation in a loop, i want to terminate the script when all the async operations are completed.. — Ghanshyam Baviskar
– Ghanshyam Baviskar, Commented Dec 29, 2018 at 17:22
Does db.collection.remove return a promise? Await will only work with functions that return promises. — Patrick Hund
– Patrick Hund, Commented Dec 29, 2018 at 17:22

T.J. Crowder · Accepted Answer · 2018-12-29 17:40:36Z

2

You've said that db.collection(...).remove(...) doesn't return a promise, but that you'll try one. Once you do:

forEach doesn't do anything with the return value of the callback, so it won't do anything to wait for the promises created by your async callback to settle.

Instead, to run them in parallel, use map and Promise.all:

await Promise.all(mobile_numbers.map(number => {
    return /*...create a promise for the remove operation here...*/;
}));

Example:

const removeNumber = number => new Promise(resolve => {
  console.log(`Removing ${number}...`);
  setTimeout(() => {
    console.log(`${number} removed`);
    resolve();
  }, Math.random() * 500);
});

async function deletor(mobile_numbers) {
    await Promise.all(mobile_numbers.map(number => {
        return removeNumber(number);
    }));
}

deletor(["111", "222", "333", "444", "555"])
    .then(() => { console.log("Done"); })
    .catch(err => { console.error("Error:", err); });

To run them in series, you can use the "Promise reduce" trick:

await mobile_numbers.reduce((p, number) => {
    return p.then(() => {
        return /*...create a promise for the remove operation here...*/
    });
}, Promise.resolve());

Example:

const removeNumber = number => new Promise(resolve => {
  console.log(`Removing ${number}...`);
  setTimeout(() => {
    console.log(`${number} removed`);
    resolve();
  }, Math.random() * 200);
});

async function deletor(mobile_numbers) {
    await mobile_numbers.reduce((p, number) => {
        return p.then(() => {
            return removeNumber(number);
        });
    }, Promise.resolve());
}

deletor(["111", "222", "333", "444", "555"])
    .then(() => { console.log("Done"); })
    .catch(err => { console.error("Error:", err); });

edited Dec 29, 2018 at 17:40

answered Dec 29, 2018 at 17:33

T.J. Crowder

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1 Comment

Ghanshyam Baviskar Over a year ago

Hi, thanks for the help, this code really worked!. you were right "forEach doesn't do anything with the return value of the callback, so it won't do anything to wait for the promises created by your async callback to settlel"

Bogdan Surai · Accepted Answer · 2018-12-30 00:29:51Z

1

1) Try this code

deletor().then(() => process.exit(0));

Async function “deletor()” returns a promise. That means you can use a method “then()” executed after “deletor()”.

Another way is using async “main()” function

async function deletor() {...}
async function main() {
    await deletor();
    process.exit(0);
 }
 main();

2) Instead of “.forEach(async ...)” you should use either

Promise.all(...)

or

for (const item of items) {
   await .....
}

First case runs parallel. Second runs serial.

edited Dec 30, 2018 at 0:29

answered Dec 30, 2018 at 0:22

Bogdan Surai

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