Javascript how to execute code after for loop completes

Question

I'm trying to work through this js/async scenario and i'm trying to know how the rest of the js world handles this.

function doStuff(callback) {

  cursor.each(function(err, blahblah) {
    ...doing stuff here takes some time
  });

  ... Execute this code ONLY after the `cursor.each` loop is finished
  callback();

EDIT

Here's a more concrete example updated using most of the suggestions below which still doesn't work.

function doStuff(callback) {

  MongoClient.connect(constants.mongoUrl, function(err, db) {

    var collection = db.collection('cases2');
    var cursor = collection.find();

    var promises = [];  // array for storing promises

    cursor.each(function(err, item) {

      console.log('inside each'); // NEVER GETS LOGGED UNLESS I COMMENT OUT THIS LINE: return Q.all(promises).then(callback(null, items));

      var def = Q.defer();        // Create deferred object and store
      promises.push(def.promise); // Its promise in the array

      if(item == null) {
        return def.resolve();
      }

      def.resolve();  // resolve the promise
    });

    console.log('items'); // ALWAYS GETS CALLED
    console.log(items);

    // IF I COMMENT THIS LINE OUT COMPLETELY, 
    // THE LOG STATEMENT INSIDE CURSOR.EACH ACTUALLY GETS LOGGED
    return Q.all(promises).then(callback(null, items));
  });
}

@VivinPaliath I think that part is irrelevant. I just loop through values of blah and push to an array. — Catfish
– Catfish, Commented Jul 31, 2014 at 16:21
How would I know when i'm at the last element of cursor.each. There is no index param from what I can tell mongodb.github.io/node-mongodb-native/api-generated/… — Catfish
– Catfish, Commented Jul 31, 2014 at 16:26
@Catfish I'd look here - github.com/kriskowal/q - plenty of examples on how to wait for multiple async things to finish before proceeding. I presume the async Mongo API you're calling within cursor.each also has the ability to invoke a callback at the end of each individual action? — Alnitak
– Alnitak, Commented Jul 31, 2014 at 16:26

dreamlab · Accepted Answer · 2014-07-31 20:47:42Z

5

without using promises or any other dependencies/libraries you can simply

function doStuff(callback) {

add a counter

    var cursor = new Array(); // init with some array data
    var cursorTasks = cursor.length;

    function cursorTaskComplete()
    {
        cursorTasks--;

        if ( cursorTasks <= 0 ) {
            // this gets get called after each task reported to be complete
            callback();
        }
    }

    for ( var i = 0; i < cursor.length; i++ ) {
        ...doing stuff here takes some time and does some async stuff

check after each async request

        ...when async operation is complete call
        cursorTaskComplete()
  }
}

edited Jul 31, 2014 at 20:47

answered Jul 31, 2014 at 16:44

dreamlab

3,38122 silver badges23 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Catfish Over a year ago

This is the way that I thought about doing it without promises.

dreamlab Over a year ago

it definitively safes a lot of processing and has less complexity.

Alnitak Over a year ago

There's no cursor.length property - there's a cursor.count() method but that is itself async!

dreamlab Over a year ago

at the time of writing there was a just an array in the question. this solution solves a general case as referred to before EDIT.

Alnitak · Accepted Answer · 2014-07-31 19:57:57Z

3

Without knowing the details of the async calls you're making within the cursor.each loop, I shall assume that you have the ability to invoke a callback each time the functions invoked therein have completed their async task:

function doStuff() {
    var promises = [];  // array for storing promises

    cursor.each(function(err, blahblah) {
        var def = Q.defer();        // create deferred object and store
        promises.push(def.promise); // its promise in the array

        call_async_function(..., def.resolve);  // resolve the promise in the async function's callback
    });

    // pass the array to Q.all, only when all are resolved will "callback" be called
    return Q.all(promises);
}

and the usage then becomes:

doStuff().then(callback)

Note how the invocation of the callback now never touches the doStuff function - that function now also returns a promise. You can now register multiple callbacks, failure callbacks, etc, all without modifying doStuff. This is called "separation of concerns".

[NB: all the above based on the Q promises library - https://github.com/kriskowal/q]

EDIT further discussion and experimentation has determined that the .each call is itself async, and gives no indication to the outside when the last row has been seen. I've created a Gist that demonstrates a resolution to this problem.

edited Jul 31, 2014 at 19:57

answered Jul 31, 2014 at 16:35

Alnitak

341k72 gold badges420 silver badges503 bronze badges

6 Comments

Catfish Over a year ago

Sorry i guess i'm NOT actually doing async stuff in my cursor.each loop. It's just the cursor.each loop that is not completing before my callback method. Not sure if that affects your answer or not.

Catfish Over a year ago

This doesn't seem to work. return Q.all(promises).then(callback); is still being executed before anything inside cursor.each is executed.

Catfish Over a year ago

I think the problem is that it takes a while to hit the callback of cursor.each so return Q.all(promises).then(callback); executes before cursor.each and promises is an empty array so it just fires the callback. I'm not sure how to circumvent this with promises.

Alnitak Over a year ago

No, in theory the promises array will already be full (because .each isn't async) by the time Q.all is called.

Alnitak Over a year ago

@Catfish re: your first comment - if there was no async code in the cursor.each loop then your original code would have worked. Re: your second comment - yes, that's how promises work. The Q.all and .then functions are themselves async - they return immediately. You can't "busy idle" in JS so you must have some mechanism to not only wait for the async stuff to finish, but to wait for doStuff to finish too. Promises provide that.

|

V31 · Accepted Answer · 2014-07-31 17:07:15Z

0

if you want to do it with the async module, you can make use of the async forEachSeries function

Code snippet:

function doStuff(callback) {

  async.forEachSeries(cursor, function(cursorSingleObj,callbackFromForEach){
      //...do stuff which takes time
      //this callback is to tell when everything gets over execute the next function
      callbackFromForEach();
  },function(){
     //over here the execution of forEach gets over and then the main callback is called
    callback();
  });
}

answered Jul 31, 2014 at 17:07

V31

7,6643 gold badges28 silver badges44 bronze badges

Comments

bhantol · Accepted Answer · 2014-07-31 19:11:59Z

-1

In my mind an elegant/ideal solution would be to have something like

 cursor.each(........).then( function() { ....your stuff});

But without that you can do this....UPDATED

http://plnkr.co/edit/27l7t5VLszBIW9eFW4Ip?p=preview

The gist of this is as shown below...notice....when

var doStuff = function(callback) {
      cursor.forEach(function(cursorStep) {
        var deferred = $q.defer();
        var promise = deferred.promise;
        allMyAsyncPromises.push(promise);
        cursorStep.execFn(cursorStep.stepMeta);
        promise.resolve;
      });

      $q.when(allMyAsyncPromises).then(callback);
}

After hitting the start button wait for few seconds...the async tasks have been simulated to finish in 5 seconds so the status will update accordingly.

Not having access to a real cursor object..I had to resort of fake cursor like and array.

edited Jul 31, 2014 at 19:11

answered Jul 31, 2014 at 17:05

bhantol

9,6368 gold badges48 silver badges84 bronze badges

2 Comments

Catfish Over a year ago

Your second solution is basically the same as @Alnitak's and the problem it doesn't work. It's returning because the code even gets inside of cursor.each.

bhantol Over a year ago

Yes similiar to @Alnitak but the minor difference is in the "starting to run" vs "completeing the async step". I created a working plnkr for you. Notice that the status "starting to run" is in the callback. plnkr.co/edit/27l7t5VLszBIW9eFW4Ip?p=preview

Collectives™ on Stack Overflow

Javascript how to execute code after for loop completes

4 Answers 4

4 Comments

6 Comments

Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

4 Comments

6 Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related