How to loop promise function flow control

Question

I'm new to use promise and Q, I'm sure I'm not doing correctly,
please give me some suggestion,

can I use fcall inside fcall? because there is a for loop I want to make sure each item image[i] process a list promise function flow..

I need the response from beginning to end, input to each promise function then pass to next flow the end return to client side,
but I don't get how to deal with the loop

var response = {};

Q.fcall(function() {
  // validate request ... 
  return response;
})
.then(function(response) {
  // save file
  for (var i = 0; i < images.length; i++) {
    Q.fcall(function() {
      // do something with images[i]
      return response;
    })
    .then(function(response) {
      // do something with images[i]
      return response;
    })
    .fail(function(error, response) {
      response.error = error;
      res.send(response);
    })
    .done(function(response) {
      return response;
    }) 
  }

  return response;  << I want this response append data from above loop if above loop all success, then to next flow save db query, if one fail then res.send(), not execute all after 
})
.then(function(response) {
  // save db query ...
  return response
})
.fail(function(error, response) {
  response.error = error;
  res.send(response);
}).done(function(response) {
  res.send(response);
});

at the moment reponse returned from `.done` in the image processing chain is lost in nowhere. Read about `Q.All`. Make an array of promises (chains that you create in `for` loop) and use `Q.All` — Kirill Slatin, Mar 21 '16 at 09:00
thanks for reply, in image process chain they need to one done then to next if use q.all able to do the same thing? — user1575921, Mar 21 '16 at 09:15
Do you need to process one image strictly after another? or just do two different things on each image? — Kirill Slatin, Mar 21 '16 at 09:17
two different things on each image but need to do first thing then next thing — user1575921, Mar 21 '16 at 09:24
Then I assume from your questions that you miss one conceptual point. A chain of promises is also a promise. So when you do `Q.fcall.then(process1).then(process2).fail(handleImageError)` it is a promise itself. And `process1` and `process2` will be fulfilled strictly one after another (assuming none of them fails). Now you can put them into array. Something like `return Q.All(images.map(function(image){ return Q.fcall(process1).then(process2).fail(...)}))` — Kirill Slatin, Mar 21 '16 at 09:27

Kirill Slatin · Accepted Answer · 2016-03-21T09:44:47.993

Given that images from the response can be processed simultaneously you can use Q.All to ensure that all images are processed successfully

Q.fcall(verifyRequest)
.then(function(){
    return Q.All(images.map(function(image){ 
        return Q.fcall(process1)
            .then(process2)
            .fail(handleImageError)
        }));
    })
.then(saveToDB)
.fail(handleRequestError)

Now all you have to do is implement the functions properly making sure data flow is correct.

Make sure that from handleImageError you return a rejection (return Q.reject();). Since fail can absorb errors if nothing returned. Actually from your code I think you don't need this handler (process each image failure) because there will be one handler that will respond with error if any of the images fails

How to loop promise function flow control

1 Answers1