Looking for help when using multiple promises at one time

Question

Here is a basic example of what I am trying to accomplish.  I have an array of things that I want to be updated using promises.  The callback of each successful promise will update a value in the thing that started the promise.

var things = [...];
for(var i=0; i < things.length; i++)
{
   getValuePromise(things[i]).then(
      function(value)	
      {			
         things[i].value = value;  //this doesn't work of course because i is not in the scope		
      }	
   );

}

I know I could do this one promise at a time by storing which thing is being updated and then chaining the promises, but I was wondering if it is possible to shoot off all the promises at once and update the values as they complete.  I just cant figure out a way tell each promise which thing to update.

This seems like something that should be easy to figure out, but I can't seem to wrap my head around it.

Answer 1

If you use forEach on your array, you'd at least have a function closure that contains index as a parameter instead of using a for loop with an unbound i:

http://msdn.microsoft.com/en-us/library/bb397509.aspx

If you want to compose a number of promises together, you may find Promise.join useful:

http://msdn.microsoft.com/en-us/library/windows/apps/br211774.aspx

I'm not sure about more specific advice, because I don't quite understand your scenario. Can you share some more details? :)

Cheers,

-Jeff

Answer 2

Hi Jeff,

I found a way to get this to work.  Here is an example where I have an array with 3 objects in it. I want to create 3 promises that return data specific to each object and then saves the data back within the object.  By using a function to wrap each promise I can access the function parameters when the promise completes.  

var objects = [
   {name:"p1", data:"abc1"},
   {name:"p2", data:"abc2"},
   {name:"p3", data:"abc3"}
]

for(var i=0; i<objects.length; i++)
{
   updateData(objects[i].name, i);
}   

function updateData(objectName, index)
{
   getUpdatedDataAsync(objectName).then(   //arbitrary promise that returns data based on parameter
      function(dataResult)
      {
         objects[index].data = dataResult;
      }
   );
}

Retrospectively, all I wanted was a way to cache variables locally for each promise so I can use them when the success function is called.