Returned variable is 'undefined' through javascript function

Question

I am having a little bit of trouble... I am trying to get a lookup result as part of a web app, but struck-upon a little trouble.

I have a function that runs of a button click:

$(function() {
    $('#bnt_fund_8y').click(function() {
        console.log(dbCheck("study_mode"));
}
});

This dbCheck function goes away and check the local DB to see what study_mode is set, in the below function:

dbCheck = function(dbSearch) {
    mydb.transaction(
        function(transaction) {
            transaction.executeSql('SELECT "'+dbSearch+'" FROM funding WHERE id = 1',[], function(transaction, results) {
                var dbResult = results.rows.item(0).study_mode;
                alert(dbResult);
                return(dbResult);
            }, errorHandler);
        });
    }

The alert display the correct variable, but he console log displays undefined. any help on this welcome...

That's an...interesting bracing style you've got there.

T.J. Crowder
– T.J. Crowder

2012-03-29 15:42:48 +00:00
Commented Mar 29, 2012 at 15:42 — T.J. Crowder
– T.J. Crowder, Commented Mar 29, 2012 at 15:42

T.J. Crowder · Accepted Answer · 2012-03-29 15:48:30Z

transaction is an asynchronous call, so your function returns (without returning any value) before the callback you give transaction occurs. The return statement in your code isn't returning anything out of dbCheck, but rather out of the callback function you've provided.

You'll need to change the function, and how you call it.

Function changes:

//  add callback parameter --v
dbCheck = function(dbSearch, callback) {
    mydb.transaction(
        function(transaction) {
            transaction.executeSql('SELECT "'+dbSearch+'" FROM funding WHERE id = 1',[], function(transaction, results) {
                var dbResult = results.rows.item(0).study_mode;
                alert(dbResult);
                // Call the callback with the result
                callback(dbResult);
            }, errorHandler);
        });
    }

Calling it:

$(function() {
    $('#bnt_fund_8y').click(function() {
        dbCheck("study_mode", function(result) {
            console.log(result);
        });
}
});

Bergi · Accepted Answer · 2012-03-29 15:42:23Z

0

dbCheck = function(dbSearch) {
    mydb.transaction(
    ...
    );
}

does not return anything, so the value you get is undefined. As transaction seems to be an aynchronous call with callback functions, the only useful thing to return would be a promise for dbResult.

answered Mar 29, 2012 at 15:42

Bergi

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Comments

Guffa · Accepted Answer · 2012-03-29 15:42:28Z

0

You can't return anything from the callback of an asynchronous call. Use a callback for the result also:

dbCheck = function(dbSearch, callback) {
  mydb.transaction(
    function(transaction) {
      transaction.executeSql('SELECT "'+dbSearch+'" FROM funding WHERE id = 1',[], function(transaction, results) {
        var dbResult = results.rows.item(0).study_mode;
        callback(dbResult);
      }, errorHandler);
    });
  });
};

Usage:

dbCheck("study_mode", function(value) {
  console.log(value);
});

answered Mar 29, 2012 at 15:42

Guffa

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Comments

KDV · Accepted Answer · 2012-03-29 15:46:22Z

0

From what I can tell,

the function dbCheck isn't returning anything at all.

mydb.transactionis the function that is actually returning something.

As everyone else is pointing out, mydb.transactionis asynchronous.

You should just add

console.log(dbResult);

into the callback being passed into mydb.transaction

answered Mar 29, 2012 at 15:46

KDV

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