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

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 

Answer 1

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.