Mastering the art of chaining promises effectively within a forEach loop in JavaScript

Question

Mastering the art of chaining promises effectively within a forEach loop in JavaScript

During my work with mongo, I encountered a need to execute an async call for each item within a loop. My goal is to carry out another command only when all promises within the loop have been fulfilled. However, currently it seems that the promises in the loop are being resolved after the code specified in the 'then' block following the loop.

I am aiming for the sequence to be:

Loop iteration promises then additional code

Instead of the current order which is:

Further code Looped 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) => {
        //Perform actions here based on the collection stats
      })
    })
  })
})
.then(() => {
  //Execute this section once all tasks above this point are completed
})
.catch((error) => {
}

Your help in resolving this issue would be greatly appreciated.

javascript mongodb promise

Answer 1

Answer №1

When working with iterables such as arrays, you have the option to use async/await in order to iterate through them using 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();
                // Perform operations based on the stats of each collection
            }
        }
    }).then(() => {
        // Execute this code after all tasks above are completed
    }).catch((error) => {
        // Handle any errors that may arise
    })

If you prefer sticking to your current .forEach() approach, you can still achieve the desired outcome by running tasks in parallel and utilizing Promise.all() to track their completion:

MongoClient.connect(connecturl).then((client) => {
    let promises = [];
    databases.forEach((val) => {
        val.collection.forEach((valcol) => {
            let p = client.db(val.databasename).stats().then((stats) => {
                // Work with the statistics of each collection here
            });
            promises.push(p);
        }); 
    });
    return Promise.all(promises);
}).then(() => {
    // Perform actions after everything above has been executed
}).catch((error) => {
    // Handle any errors within this block
});

Answer 2

When working with iterables such as arrays, you have the option to use async/await in order to iterate through them using 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();
                // Perform operations based on the stats of each collection
            }
        }
    }).then(() => {
        // Execute this code after all tasks above are completed
    }).catch((error) => {
        // Handle any errors that may arise
    })

If you prefer sticking to your current .forEach() approach, you can still achieve the desired outcome by running tasks in parallel and utilizing Promise.all() to track their completion:

MongoClient.connect(connecturl).then((client) => {
    let promises = [];
    databases.forEach((val) => {
        val.collection.forEach((valcol) => {
            let p = client.db(val.databasename).stats().then((stats) => {
                // Work with the statistics of each collection here
            });
            promises.push(p);
        }); 
    });
    return Promise.all(promises);
}).then(() => {
    // Perform actions after everything above has been executed
}).catch((error) => {
    // Handle any errors within this block
});

Mastering the art of chaining promises effectively within a forEach loop in JavaScript

Answer №1

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