How to return value from async method? - asynchronous

Question

I need to get value from asynchronous query. I tried to get my code to return it, but it still works wrong. Firstly it write in console "undefined", then it write "19" - it's correct value. So, where is an error?
My code:
var Back = Parse.Object.extend("Back");
var query = new Parse.Query(Back);
var LastSerial;
query.get("ghxbtU2KSl").then(function(result){
LastSerial=result.get("SerialNumber");
console.log(LastSerial);
return LastSerial
});
console.log(LastSerial);

Answer 1

As someone already mentioned in the comments, the query will be run asynchronously which means Lastserial will not have been assigned a value when you print it to the console. Everything that depends on the value of Lastserial will need be either nested inside the callback or you may take a look at promises in series promises in parse docs I edited your example to visualize what this means:
var Back = Parse.Object.extend("Back");
var query = new Parse.Query(Back);
var LastSerial;
query.get("ghxbtU2KSl").then(function(result){
//async
LastSerial=result.get("SerialNumber");
//this will print the value correctly
console.log(LastSerial);
return LastSerial
});
//this will be run without waiting for the query to finish, hence print undefined
console.log(LastSerial);

Answer 2

You cannot "return" a value from a .then clause in a Promise. The then clause will run sometime in the future after the code that created it has already exited. The way to handle this is call a function from the then clause that processes the returned data.
Alternatively, you can "chain" your Promises.
var Back = Parse.Object.extend("Back");
var query = new Parse.Query(Back);
var LastSerial;
var p0;
var p1;
p0 = query.get("ghxbtU2KSl");
p1 = p0.then(function(result){
//async
LastSerial=result.get("SerialNumber");
//this will print the value correctly
console.log(LastSerial);
return LastSerial
});
p1.then(function(result){
// Do stuff with result, which is LastSerial
console.log(result);
return 0
});
//this will be run without waiting for the query to finish, hence print undefined
console.log(LastSerial);