angularjs - Cordova File Plugin and Angular - Read JSON -

i try make user service connect facebook. i've try localstorage plugin, when memory low, ios delete files , cause white screen.

so, now, use cordova file plugin , want implement user service store user datas.

this first app, target ios , android, device iphone 5s, ios 8.3 , develop in macos.

in config.xml, i've :

<preference name="iospersistentfilelocation" value="library"/> 

and, user service :

myapp.service('userservice', function($cordovafile) {    var setuser = function(user_data) {     $cordovafile.writefile(cordova.file.datadirectory, "myapp_user", json.stringify(json), true)   }    var userisloggedin = function() {     var user = getuser()     return user && user.authresponse && user.authresponse.userid != null;   }    var userisnotloggedin = function() {     var user = getuser()     return user && user.authresponse && user.authresponse.userid == null;   }    var getuser = function() {     return $cordovafile.readastext("myapp_user");   }    return {     getuser: getuser,     setuser: setuser,     userisloggedin: userisloggedin,     userisnotloggedin: userisnotloggedin   }  }) 

in facebook connect, i've recover data (it's work localstorage, problem not here)

    userservice.setuser({       authresponse: authresponse,       name: profileinfo.name,       user_id: profileinfo.id,       first_name: profileinfo.first_name,       picture: profileinfo.picture.data.url,       ionic_id: ionicuser.id     }) 

so, login facebook work charm, when want read json data, can't ! have white screen , error : userid not defined !

can me read json file ?

thanks !