CartoDB SQL and returning values

Question

Right I've got this JS function

 function AreaCompare(sql, UIDfield, Geogname, Table, sourceName, sourceID, destName, destID){
 CompareObj = {};
 CompareObj.sName = sourceName;
 CompareObj.sID = sourceID;
 CompareObj.dName = destName;
 CompareObj.dID =destID;
 sql.execute("select ST_Area(the_geom::geography)/1000000 as SA FROM "+Table+" where "+UIDfield+"="+sourceID)
 .done(function(data) {
 CompareObj.sArea=JSON.stringify(data.rows[0].sa);
 sql.execute("select ST_Area(the_geom::geography)/1000000 as DA FROM "+Table+" where "+UIDfield+"="+destID)
 .done(function(data) {
 CompareObj.dArea=JSON.stringify(data.rows[0].da);
 alert(JSON.stringify(CompareObj));
 return CompareObj;
 })
 })
}

It appears to work correctly and the alert box flashes up with a readable version of the object that is as expected.

however when I examine the returned object it is undefined, essentially empty/null.

What trick am I missing a trick here? It originally thought it was something related to scope and the done function.

I did allot of research and I realised I needed to nest the CartoDB SQL statements, but that didn't fix the problem.

Any suggestions?

• toms: Have you tried to declare CompareObj as a global variable?
• Alex Leith: Also, the beauty of browser based code is debugging. Run this is Chrome, or Firefox, and put a breakline at the return statement. Then when you run the function, it will pause there and you can inspect the variable to see if it contains what you expect it to.
• Omar Sarhan: @toms yep I tried that, some success, It will give me the object but will not contain the key/value for the values set inside the 'done' functions. I think this is because CartoDB uses an AJAX for the SQL queries and java script executes the other code while waiting on a response.

Answer 1

Maybe this pattern will help? If alert on .done is showing the expected result, you should be able to pass CompareObj to a new function (also from within the .done function), and evaluate the global CompareObj from there. No need to worry about asynchronous response in that case, since the new function won't execute until AJAX is done.

// declare this as a global variable
CompareObj = {};
function AreaCompare(sql, UIDfield, Geogname, Table, sourceName, sourceID, destName, destID){
 CompareObj.sName = sourceName;
 CompareObj.sID = sourceID;
 CompareObj.dName = destName;
 CompareObj.dID =destID;
 sql.execute("select ST_Area(the_geom::geography)/1000000 as SA FROM "+Table+" where "+UIDfield+"="+sourceID)
 .done(function(data) {
 CompareObj.sArea=JSON.stringify(data.rows[0].sa);
 sql.execute("select ST_Area(the_geom::geography)/1000000 as DA FROM "+Table+" where "+UIDfield+"="+destID)
 .done(function(data) {
 CompareObj.dArea=JSON.stringify(data.rows[0].da);
 // if you are getting what you want here, pass the object to a new function and do something with it
 alert(JSON.stringify(CompareObj));
 // return CompareObj;
 useResult(CompareObj);
 })
 })
}
// new function to use the result
function useResult(CompareObj) {
 // do something with the result here...
}

• Omar Sarhan: Thanks Toms, you are a great help. My code is a modified version of yours now: dropbox.com/s/emvzlkq44gbtc29/Compare.js?dl=0 Now line 21 displays the correct answer as an alert, but in my main script block if I call: AreaCompare(sql, UIDfield, Geogname, Table, sourceName, sourceID, destName, destID) then call alert(GetBiggest()) Ill get an undefined` back. In fact if I have both alerts set up I get the undefined before the correct alert generated from within the AreaCompare function (as per line 21) I`m no whizz at programming but I should be able to do this. :/