How to use defer() the right way?

Question

How do I use defer correctly? I've got two functions here, in which one defer is used correctly? In which one it is used incorrectly? And why resp. why not?

First example:

getFoo1: function() {
  var dfd = $q.defer();

  db.allDocs({include_docs: true}, function(err, response) {
    if(err) {
      console.log(err);
      dfd.reject(err);
    } else {
      var qcs = [];
      for(var i=0; i < response.total_rows; i++) {
        if(response.rows[i].doc.type == 'bar') {

          var qc = {id: response.rows[i].doc._id,
            isFoo: true
          };

          oneFunction(qc)
          .then(anotherFunction(qc))
          .then(qcs.push(qc));
        }
      }
      dfd.resolve(qcs);
    }
  });
  return dfd.promise;
},

Second example:

getFoo2: function() {
  var dfd = $q.defer();

  db.allDocs({include_docs: true}, function(err, response) {
    if(err) {
      dfd.reject(err);
    } else {
      dfd.resolve(response);
    }
  });

  return dfd.promise
  .then(function(response) {
    var foo = [];
    for(var i=0; i < response.total_rows; i++) {
      if(response.rows[i].doc.type == 'bar') {
        var qc = {id: response.rows[i].doc._id,
          isFoo: true
        };

        return oneFunction(qc)
        .then(anotherFunction(qc))
        .then(foo.push(qc));
      }   
    }
  }, function(err){
     console.log(err);
  });
},

The oneFunction does nothing asynchronously.

The anotherFunction does something asynchronously (retrieving data from an external database).

EDIT: Thanks to @Bergi the correct function should look like this:

getFoo3: function() {
  var dfd = $q.defer();

  db.allDocs({include_docs: true}, function(err, response) {
    if(err) {
      dfd.reject(err);
    } else {
      dfd.resolve(response);
    }
  });

  return dfd.promise
  .then(function(response) {
    var foos = [];

  for (var i=0; i < response.total_rows; i++) {
    if (response.rows[i].doc.type == 'bar') {
      var qc = {id: response.rows[i].doc._id,
        isFoo: true
      };
      var foo = oneFunction(qc);
      foos.push(foo);
    }
  }

  var promises = foos.map(anotherFunction); // make a promise for each foo
  return $q.all(promises);

  }, function(err){
     console.log(err);
  });
},

to `then` you should pass reference to function, but instead you instant call function like `anotherFunction(qc)`. So seems in both it used incorrectly :-) — Grundy, Jul 01 '15 at 13:41
Could you elaborate on your comment, please? At which `then` part? How should the function looks like instead, @Grundy? — thadeuszlay, Jul 01 '15 at 13:46
if you explain what you expected, i can try explain how it should. Anyway i mean `.then(anotherFunction(qc))` should be like `.then(function(){anotherFunction(qc)})` until `anotherFunction` not return function — Grundy, Jul 01 '15 at 13:48
How does `anotherFunction()` gets it's parameter `qc`, @Grundy? — thadeuszlay, Jul 01 '15 at 13:51
but here a ugly sample, because in js variable scope on function level, so always passed last value — Grundy, Jul 01 '15 at 13:54
so here seems more right: `.then((function(a){return function(){anotherFunction(a)}})(qc))` :-) or simple `.then(anotherFunction.bind(null, qc))` — Grundy, Jul 01 '15 at 13:56
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/82098/discussion-between-thadeuszlay-and-grundy). — thadeuszlay, Jul 01 '15 at 14:07
Why it's not `.then(function() {return function(qc){anotherFunction(qc)}})`, @Grundy — thadeuszlay, Jul 01 '15 at 14:18
Both examples are incorrect. You're using [tag:pouchdb] in your example, Pouch already returns promises - so the correct usage pattern would be to use the promises it returns and work with them and then wrap it in a `$q.when`. I can elaborate in chat. — Benjamin Gruenbaum, Jul 01 '15 at 17:29
@BenjaminGruenbaum: Yeah, sure. :) Please elaborate that in chat. — thadeuszlay, Jul 01 '15 at 17:30

score 2 · Accepted Answer · edited May 23 '17 at 12:29

You've used $q.defer correctly in the second example^[1] - creating a promise for the response of the db.allDocs asynchronous call (and nothing else). Altough pochdb seems to already return promises, as @Benjamin mentions in the comments, so it's unnecessary (but not wrong).

The first example is just a mess, convoluting the construction of the promise with the error logging and that ominous loop.

_{1: Except for dfd.promise(), which is not a function but a property. Go for dfd.promise.then(…)}.

However, that loop is a very different topic, and seems to be wrong in both snippets. Some points:

In the second snippet, your return from the callback function in the body of the loop, right on the first iteration that fulfills the predicate.
If oneFunction(qc) is not asynchronous, it doesn't need to (read: shouldn't) return a promise - or if it does not, that .then(…) call is wrong.
anotherFunction(qc) seems to return a promise (it's asynchronous as you say). But you must not pass that promise to .then(), a callback function is expected there!
Same thing for foo.push(qc) - it's not a callback function that you would pass to .then().
After all, you are doing something asynchronous in that loop. Here, you have to use Promise.all now!

If I had to bet, I'd say you need

.then(function(response) {
  var foos = [];
  for (var i=0; i < response.total_rows; i++) {
    if (response.rows[i].doc.type == 'bar') {
      var qc = {id: response.rows[i].doc._id,
        isFoo: true
      };
      var foo = oneFunction(qc);
      foos.push(foo);
    }
  }
  var promises = foos.map(anotherFunction); // make a promise for each foo
  return $q.all(promises);
})

response.rows can be `map`d since it's an array, other than that looks good :) — Benjamin Gruenbaum, Jul 01 '15 at 18:35
Yeah, `filter`ed and `map`d, but I wanted to stay close to the original code. — Bergi, Jul 01 '15 at 18:40
@thadeuszlay: No, for the predicate `response.rows[i].doc.type == 'bar'` a standard [Array `filter`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter) will suffice. — Bergi, Jul 07 '15 at 14:07

How to use defer() the right way?

1 Answers1