从 javaScript 函数中的 WebSQL 查询返回 COUNT

我想返回数据库中特定表的行数WebSQL里面一个javascript功能。下面是我的代码。

function getCustomerCount(){    
    var count = 0;  
    db.transaction(function(tx) {   
    tx.executeSql('SELECT * FROM contacts', [], function(tx, results) {
             count = results.rows.length;   
        });
    });
    return count;
}

我是新来的WebSQL并且比较陌生javascript also.
有什么建议么？

你不能这样做：

function getCustomerCount(){    
    var count = 0;  
    db.transaction(function(tx) {   
    tx.executeSql('SELECT * FROM contacts', [], function(tx, results) {
             // this function is called when the executeSql is ended
             count = results.rows.length;   
        });
    });
    // This returns always 0!
    return count;
}

您必须像这样使用回调：

function getCustomerCount( callback ){    
    var count = 0;  
    db.transaction(function(tx) {   
    tx.executeSql('SELECT * FROM contacts', [], function(tx, results) {
             // this function is called when the executeSql is ended
             count = results.rows.length;
             callback( count );   // <-- call the callback when is done   
        });
    });
}

并像这样使用它：

getCustomerCount( function( count ) {
    // do what you want with the count
    console.log( 'The number of rows is: '+ count );
});