javascript - getting data from async function - Stack Overflow

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i have this code:

    function getData(){
        db.transaction(function(tx){
            tx.executeSql('SELECT * from q', [], function(tx, result){
                var q = [];
                for (var i=0; i < result.rows.length; i++) {
                    q.push(result.rows.item(i));
                };
                console.log(q.length);  // 3
                returnData(q);
            });
        });
    }

    function returnData(data){
        console.log(data.length); // 3
        return data;
    }

   var q = getData(); // undefined

and it don't work as expected (it don't return anything). A assume that happened, because db.transaction work asynchronous, but i'm using callback to return data. Can somebody explain why it doesn't work and how to fix that?

i have this code:

    function getData(){
        db.transaction(function(tx){
            tx.executeSql('SELECT * from q', [], function(tx, result){
                var q = [];
                for (var i=0; i < result.rows.length; i++) {
                    q.push(result.rows.item(i));
                };
                console.log(q.length);  // 3
                returnData(q);
            });
        });
    }

    function returnData(data){
        console.log(data.length); // 3
        return data;
    }

   var q = getData(); // undefined

and it don't work as expected (it don't return anything). A assume that happened, because db.transaction work asynchronous, but i'm using callback to return data. Can somebody explain why it doesn't work and how to fix that?

• javascript
• asynchronous
• web-sql

Share Improve this question Follow asked Aug 25, 2011 at 21:21 nuklnukl 10.5k1515 gold badges4444 silver badges5858 bronze badges

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2 Answers 2

Sorted by: Reset to default 7

The standard way to do this is to include your own callback, like this:

function getData(callback){
    db.transaction(function(tx){
        tx.executeSql('SELECT * from q', [], function(tx, result){
            var q = [];
            for (var i=0; i < result.rows.length; i++) {
                q.push(result.rows.item(i));
            };
            console.log(q.length);  // 3
            callback(returnData(q));
        });
    });
}

function returnData(data){
    console.log(data.length); // 3
    return data;
}

getData(function(q) {
    /* do something with q */
});

You don't return the result of any async action, instead you listen for it.

In your code returnData does return the data, but the you dont do anything with the result, it's discarded. Instead, you should use your own callback.

function getData(callback){
    db.transaction(function(tx){
        tx.executeSql('SELECT * from q', [], function(tx, result){
            var q = [];
            for (var i=0; i < result.rows.length; i++) {
                q.push(result.rows.item(i));
            };
            console.log(q.length);  // 3
            callback(q);
        });
    });
}

var q;
getData(function(data) {
    console.log(data.length); // 3
    console.log(data);
    doStuffWith(data);
    q = data;
});

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