Javascript Array strange behavior. Global declaration is unable to hold value

Question

I have the following code in one of my function. I have an array 'arr' which is working correctly when used inside if{}. But its now working when using outside it. Can anyone point me what I am missing.

 function runQueries()
 {
    var arr = new Array;

    db.transaction (function (transaction) 
      {
        var sql = "SELECT * FROM incomecategory";

        transaction.executeSql (sql, undefined, 
        function (transaction, result)
        {
          if (result.rows.length)
          {
            for (var i = 0; i < result.rows.length; i++) 
            {
              var row = result.rows.item (i);
              var categoryname = row.categoryname;
              arr[i] = categoryname;
            }
                              //alert(arr[0]); // It works


          }  
          else
          {

          }
        }, error);

      });
       //alert (arr[0]); // It doesn't work. 

 }

open up the error console of your browser, and see if you get any javascript errors — Sam I am says Reinstate Monica
– Sam I am says Reinstate Monica, Commented Oct 3, 2013 at 20:46
Not sure what framework this webSQL-ish code runs under, but they're often asynchronous. That transaction function isn't actually running until after the outer function exits (and so, after the alert call). — Paul Roub
– Paul Roub, Commented Oct 3, 2013 at 20:47
That's because when you alert(arr[0]) your array is STILL empty. if you alert("a") inside and alert("b") outside, it will alert "b" first and "a" later, when your sql finishes executing. — SparK
– SparK, Commented Oct 3, 2013 at 20:48

webduvet · Accepted Answer · 2013-10-03 20:46:37Z

4

It's asynchronous behavior. Your alert at the bottom of the code is probably executed before the database query.

answered Oct 3, 2013 at 20:46

webduvet

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2 Comments

SparK Over a year ago

/agreed. What you need to do is call a callback function that receives your array as a parameter (and does whatever you need it to do with it) inside your "if".

user2844346 Over a year ago

thanks. I understood the problem. Is there any way to delay use of arr so that its used only after database query? using a call back function is an option. but in my case , it will be very complicated. this arr is supposed to hold categories. then I have another function ready which runs another query to hold totals of each categories in another array. then I have third function which calculates percentage. finally fourth function would create graph using values held in these series of arrays.

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