如何正确使用defer（）？

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);
  });
},

Answer 1

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);
})