How to properly chain promises inside of a forEach loop in Javascript

Question

I'm using mongo and need to do an async call for each item inside of a loop. I'd like to execute another command once all of the promises inside of the loop have completed, but so far the promises in the loop seem to be completing after the code that's in the then that's after the loop.

Essentially i'd like the order to be

Loop promises then other code

instead of what it is now which is

other code Loop promises

MongoClient.connect(connecturl)
.then((client) => {
  databases.forEach((val) => {
    val.collection.forEach((valcol) => {
      client.db(val.databasename).stats() //(This is the async call)
      .then((stats) => {
        //Do stuff here with the stats of each collection
      })
    })
  })
})
.then(() => {
  //Do this stuff after everything is finished above this line
})
.catch((error) => {
}

Any assistance would be appreciated on this one.

jfriend00 · Accepted Answer · 2018-02-04 01:55:18Z

4

Assuming the things you are using .forEach() on are iterables (arrays or something like that), you can use async/await to serialize a for/of loop:

    MongoClient.connect(connecturl).then(async (client) => {
        for (let db of databases) {
            for (let valcol of db.collection) {
                let stats = await client.db(db.databasename).stats();
                // Do stuff here with the stats of each collection
            }
        }
    }).then(() => {
        // Do this stuff after everything is finished above this line
    }).catch((error) => {
        // process error
    })

If you wanted to stick with your .forEach() loops, you could make it all work if you did things in parallel and used Promise.all() to know when it's all done:

MongoClient.connect(connecturl).then((client) => {
    let promises = [];
    databases.forEach((val) => {
        val.collection.forEach((valcol) => {
            let p = client.db(val.databasename).stats().then((stats) => {
                // Do stuff here with the stats of each collection
            });
            promises.push(p);
        }); 
    });
    return Promise.all(promises);
}).then(() => {
    // Do this stuff after everything is finished above this line
}).catch((error) => {
    // process error here
});

edited Feb 4, 2018 at 1:55

answered Feb 4, 2018 at 1:40

jfriend00

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6 Comments

Allen Over a year ago

Top level await wasn't ready yet, then callback needs to be marked as async.

jfriend00 Over a year ago

@Xlee - You're right. I added an async to the first .then() handler and removed the outer async wrapper function.

Milos Over a year ago

I'm using your forEach solution where you push the promises to the array and it seems to work exactly how I want to! I think it screwed something up though where i'm grabbing the database stats with "stats" rather than the collection stats.

Allen Over a year ago

@Milos The reason yours doesn't work is your first then doesn't return a Promise, thus treated as a synchronous call. I like the solution using Promise.all, since it parallelizes your async tasks. You can also use reduce function to achieve sequential async tasks though.

Milos Over a year ago

@Xlee by the first do you mean the mongoconnect needs to return a promise or am i reading into your comment wrong? If you could explain it a little further i'd appreciate it very much. Thanks.

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