Vows and Q - Wrapping up an extensive array of tasks using a forEach loop

Question

Vows and Q - Wrapping up an extensive array of tasks using a forEach loop

I'm feeling a bit lost on the best way to approach this task - hopefully, my explanation is clear.

In my forEach function, I am iterating through some JS object data and running a function for each item:

found.forEach(function(item) {
      processData(item['userID']);
});

Inside the processData function, I am making a MongoDB find() call.

var processData = function(userIDSelected) {

    User.find({_id: userIDSelected},
           {gender: 1, country:1}, function(req, foundUser) {
            processUserInfo(foundUser[0]['gender']);
    });
}

The challenge here is how to ensure that everything in the forEach loop completes before moving on, especially since each call triggers processUserInfo one by one.

I've explored using the Q library and Q.all, but it doesn't seem to work as intended.

Is there a specific Q function that can help me wait for the entire chain of operations to finish?

Thank you

javascript mongodb promise q

Answer 1

Answer №1

Q.all:

The function Q.all returns a promise that will be fulfilled with an array containing the fulfillment value of each promise, or rejected with the same reason as the first promise to be rejected.

Alternatively, you can use Q.allSettled:

Q.allSettled returns a promise that is fulfilled with an array of promise state snapshots only after all the original promises have settled, either being fulfilled or rejected.

To implement this, follow these steps:

Adjust the processData function and possibly your MongoDB call so that processData returns a promise for the asynchronous operation. If MongoDB doesn't return a promise automatically, refer to relevant resources on converting callback-based APIs to promises.
Replace forEach with map to get an array of resulting promises.
Apply Q.all or Q.allSettled on the array of promises.

If User.find returns a promise without a specified callback, step #1 should look something like this:

var processData = function(userIDSelected) {

    return User.find(
           {_id: userIDSelected},
           {gender: 1, country:1}
    ).then(function(req, foundUser) {
        return processUserInfo(foundUser[0]['gender']);
    });
};

If not, you can create the promise manually using Q.defer:

var processData = function(userIDSelected) {
    var d = Q.defer();
    User.find(
           {_id: userIDSelected},
           {gender: 1, country:1},
           function(req, foundUser) {
        processUserInfo(foundUser[0]['gender']);
        d.resolve(/*...data could go here...*/); 
    });
    return d.promise;
};

Steps 2 and 3 would look like:

Q.all(found.map(function(item) {
    return processData(item);
}))
.then(...)
.catch(...);

If processData only uses its first argument, you can simplify it further:

Q.all(found.map(processData))
.then(...)
.catch(...);

Ensure that processData ignores extra arguments when using map.

Answer 2

Q.all:

The function Q.all returns a promise that will be fulfilled with an array containing the fulfillment value of each promise, or rejected with the same reason as the first promise to be rejected.

Alternatively, you can use Q.allSettled:

Q.allSettled returns a promise that is fulfilled with an array of promise state snapshots only after all the original promises have settled, either being fulfilled or rejected.

To implement this, follow these steps:

Adjust the processData function and possibly your MongoDB call so that processData returns a promise for the asynchronous operation. If MongoDB doesn't return a promise automatically, refer to relevant resources on converting callback-based APIs to promises.
Replace forEach with map to get an array of resulting promises.
Apply Q.all or Q.allSettled on the array of promises.

If User.find returns a promise without a specified callback, step #1 should look something like this:

var processData = function(userIDSelected) {

    return User.find(
           {_id: userIDSelected},
           {gender: 1, country:1}
    ).then(function(req, foundUser) {
        return processUserInfo(foundUser[0]['gender']);
    });
};

If not, you can create the promise manually using Q.defer:

var processData = function(userIDSelected) {
    var d = Q.defer();
    User.find(
           {_id: userIDSelected},
           {gender: 1, country:1},
           function(req, foundUser) {
        processUserInfo(foundUser[0]['gender']);
        d.resolve(/*...data could go here...*/); 
    });
    return d.promise;
};

Steps 2 and 3 would look like:

Q.all(found.map(function(item) {
    return processData(item);
}))
.then(...)
.catch(...);

If processData only uses its first argument, you can simplify it further:

Q.all(found.map(processData))
.then(...)
.catch(...);

Ensure that processData ignores extra arguments when using map.

Answer 3

Answer №2

From the way you have written your code, it seems like userID is expected to be an ObjectId from MongoDB.

If this assumption holds true, then the function will execute successfully only if the variable found contains at least one element. Otherwise, the users will be left waiting indefinitely for a response from the server.

processData(
  // Retrieve an object 
  // {
  //   $in: [ObjectId, ObjectId, ...]
  // }
  // 
  found.reduce(
    function(query, user) {
      return query.$in.push(user.userID), query;
    },
    {$in: []}
  )
);

You can find more information about the $in operator in MongoDB here.

Answer 4

From the way you have written your code, it seems like userID is expected to be an ObjectId from MongoDB.

If this assumption holds true, then the function will execute successfully only if the variable found contains at least one element. Otherwise, the users will be left waiting indefinitely for a response from the server.

processData(
  // Retrieve an object 
  // {
  //   $in: [ObjectId, ObjectId, ...]
  // }
  // 
  found.reduce(
    function(query, user) {
      return query.$in.push(user.userID), query;
    },
    {$in: []}
  )
);

You can find more information about the $in operator in MongoDB here.

Vows and Q - Wrapping up an extensive array of tasks using a forEach loop

Answer №1

Answer №2

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