data store problem

[proofmoore]

var infoStore = new Ext.data.Store({
proxy:new Ext.data.HttpProxy({
url:'login.jsp?op=getUserData'
}),

reader:new Ext.data.JsonReader(
{
root:'data'
},
[
{
name:'u',
mapping:'name',

},

{
name:'m',
mapping:'mail'
},

{
name:'p',
mapping:'PASSWORD'
},
])
});


infoStore.on('load',function(){
alert( infoStore.data.u);
});

----JSON from firebug:
{'data': [{'u':'myname','m':'yahoo','p':'7875775'}]}

the alert function here out empty string. i want to out 'myname' here