How to return value from an asynchronous callback function

Question

This question is asked many times in SO  But still I can t get stuff   I want to get some value from callback  Look at the script below for clarification    function foo address             google map stuff       geocoder geocode     address   address   function results  status              results 0  geometry location     I want to return this value                     foo      result should be results 0  geometry location  value   If I try to return that value just getting  undefined   I followed some ideas from SO  but  still fails    Those are   function foo address       var returnvalue          geocoder geocode     address   address   function results  status            returnvalue   results 0  geometry location              return returnvalue     foo      still undefined

User · Accepted Answer

This is impossible as you cannot return from an asynchronous call inside a synchronous method.

In this case you need to pass a callback to foo that will receive the return value

function foo(address, fn){
  geocoder.geocode( { 'address': address}, function(results, status) {
     fn(results[0].geometry.location); 
  });
}

foo("address", function(location){
  alert(location); // this is where you get the return value
});

The thing is, if an inner function call is asynchronous, then all the functions 'wrapping' this call must also be asynchronous in order to 'return' a response.

If you have a lot of callbacks you might consider taking the plunge and use a promise library like Q.

User · Answer

If you happen to be using jQuery  you might want to give this a shot  http   api jquery com category deferred-object   It allows you to defer the execution of your callback function until the ajax request  or any async operation  is completed  This can also be used to call a callback once several ajax requests have all completed

User · Answer

It makes no sense to return values from a callback  Instead  do the  foo    work you want to do inside your callback   Asynchronous callbacks are invoked by the browser or by some framework like the Google geocoding library when events happen   There s no place for returned values to go  A callback function can return a value  in other words  but the code that calls the function won t pay attention to the return value

[javascript] How to return value from an asynchronous callback function?

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