javascript

Chapter 21: Upcoming API s
Database Storage
HTML 5 introduces a client - side database storage system that is accessible from JavaScript. The first
browser to implement this feature was Safari 3.1; no other browsers have implemented it at the time of
this writing. To start using client - side database storage, you ’ ll need to call the openDatabase() method
on the window object. This method accepts four arguments: the database name, the database version, a
display name, and the estimated size of the database in bytes. If the database already exists, then
openDatabase() returns a Database object representing the existing database; if the database doesn ’ t
already exist, openDatabase() first creates the new database and then returns a Database object for it.
Here is an example:
var db = window.openDatabase(“Test DB”, “1.0”, “My Test Database”, 200000);
The primary method of interacting with a database is the transaction() method. This method accepts
three arguments: a transaction callback that is executed when the system is ready to work with the
database, an optional error callback, and an optional success callback. All database operations are
executed asynchronously, which is why each argument to transaction() is a function.
A transaction callback function receives a SQLTransaction object as its only argument. This object has
only one method, executeSql() , which accepts four arguments: the SQL string to execute, an optional
array of parameters to embed in the SQL, an optional success callback, and an optional error callback.
The success callback receives two arguments: a SQLTransaction object and a SQLResultSet object
containing any results. The error callback receives a SQLTransaction object and an error object
indicating that the error that occurred.
To begin using a client - side database, you ’ ll need to create a table as follows:
db.transaction(function(transaction){
});
transaction.executeSql(“CREATE TABLE Messages (id REAL UNIQUE, msg TEXT)”, [],
function(transaction, results){
//database was created
},
function(transaction, error){
//database wasn’t created
}
);
This code creates a simple table named Messages that has two fields: the primary key id and msg to
hold some text. The second argument is an empty array because it ’ s not necessary for the completion of
the database operation. If the creation is successful, then the success callback is executed. There won ’ t be
any results for this operation, so the callback can just be used to indicate that the database was created as
expected. If the database isn ’ t created, then the error callback is executed and you can use the error
argument to determine what happened.
Since there isn’t a way to simply detect the existence of a database table, you may
need to first run a query against the table and, if an error occurs, assume that the
table doesn’t exist.
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