How to pass a local variable to a callback while getting something returned by the calling function?

Question

I have a problem with a async callback in a loop. I call a asynchronous function on one of Window's file objects to get further informations about it (getImagePropertiesAsync()) and save these to a database to make further access faster. I need the values that are returned by that functions as well as the value i had when I called getImagePropertiesAsync().

for (var i = 0; i < picsGiven.length; i++) {
  picsGiven[i].properties.getImagePropertiesAsync().then(function(prop){
    settings.values["geoLat" + i2] = props.latitude;
    settings.values["geoLon" + i2] = props.longitude;
    settings.values["geoDate" + i2] = props.dateTaken;
    i2++;
  })
}

Because of the asynchronous execution, the pairs three values got saved in the wrong order. I already worried about that problem by the time I was writing that. Next try.

for (var i = 0; i < picsGiven.length; i++) {
  picsGiven[i].properties.getImagePropertiesAsync().then((function(prop, i2){
    settings.values["geoLat" + i2] = props.latitude;
    settings.values["geoLon" + i2] = props.longitude;
    settings.values["geoDate" + i2] = props.dateTaken;
  })(undefined, i))
}

Now, I got i correctly, but props is undefined. (Logically).

How can I possibly get i correctly while preserving props? Thanks in advance!

possible duplicate of [Javascript closure inside loops - simple practical example](http://stackoverflow.com/questions/750486/javascript-closure-inside-loops-simple-practical-example) — Felix Kling, Aug 02 '13 at 07:21

score 1 · Accepted Answer · answered Aug 02 '13 at 07:20

(Untested) something like this:

function mk_handleImagePropertiesAsync(ival) {
  return function(prop) {
    settings.values["geoLat" + ival] = prop.latitude;
    settings.values["geoLon" + ival] = prop.longitude;
    settings.values["geoDate" + ival] = prop.dateTaken;
  }
}

for (var i = 0; i < picsGiven.length; i++) {
  picsGiven[i]
    .properties
    .getImagePropertiesAsync()
    .then(mk_handleImagePropertiesAsync(i));
}

Scoping issue! The for-loop does not introduce a new block-scope as in C/C++. The inline function in your loop will always get the same value of i2 (its initial value!)

Thanks for the fast answer. I'm trying now, but running this might take some time... — Martin, Aug 02 '13 at 07:28

score 0 · Answer 2 · answered Aug 02 '13 at 07:36

0

try this

for (var i = 0; i < picsGiven.length; i++) {
       picsGiven[i].properties.getImagePropertiesAsync().then(function(i){
           return function(prop){
               //your code  here you can use i
           }
       }(i));
    }

answered Aug 02 '13 at 07:36

zqillyc

21
3

How to pass a local variable to a callback while getting something returned by the calling function?

2 Answers2