JavaScript .map on an array and removing items if condition satisfied

Question

I have an array queue that I push objects to it when they are modified. If the user presses save, then I will loop through the queue and apply the appropriate API call for them.

If the API call goes through successfully, I want to remove the item from the queue, otherwise keep it inside and notify the user that some items were not successfully saved. I currently have this (in AngularJS)

var unsuccessfulItems = [];
var promise = queue.map(function(item) {
    var defer = $q.defer();
    myCallFunction( item
           , function( response ) {} // Success
           , function( response ) {  // Error
               unsuccessfulItems.push(item);
           }
    )
    defer.resolve();
    return defer.promise;
})
// Once all items have been processed
$q.all( promise ).then( function() {
    queue = unsuccessfulItems;
});

Is there a better way of doing this?

Answer 1

You're already using promises, you might want to do it end-to-end. Also, you're resolving the promise too early.

Assuming the suboptimal case where you don't want to promisify myCallFunction itself, you should still promisify it.

function myCall(item){
    var d = $q.defer();
    myCallFunction(item,function(r){ d.resolve({val:r,item:item});}
                       ,function(r){ d.reject(r);});
    return d.promise;
}

Note, we are resolving the defer after the asynchronous function is done, not before it.

Now, we need to implement a "Settle" function, that resolves when all promises are done no matter what. This is like $q.all but will wait for all promises to resolve and not fulfill.

function settle(promises){
     var d = $q.defer();
     var counter = 0;
     var results = Array(promises.length);
     promises.forEach(function(p,i){ 
         p.then(function(v){ // add as fulfilled
              results[i] = {state:"fulfilled", promise : p, value: v};
         }).catch(function(r){ // add as rejected
              results[i] = {state:"rejected", promise : p, reason: r};
         }).finally(function(){  // when any promises resolved or failed
             counter++; // notify the counter
             if (counter === promises.length) {
                d.resolve(results); // resolve the deferred.
             }
         });
     });
}

This sort of settle function exists in most promise implementations but not in $q. We could have also done this with rejections and $q.all, but that would mean exceptions for flow control which is a bad practice.

Now, we can settle:

 settle(queue.map(myCall)).then(function(results){
     var failed = results.filter(function(r){ return r.state === "rejected"; });
     var failedItems = failed.map(function(i){ return i.value.item; });
 });