You can use a closure:
var a = 10;
PageMethods.SaveData(
JSON.stringify(basicInfo),
function(response) {
SaveSuccessCallback(response);
ShowEdit(a);
}, SaveFailedCallback);
function SaveSuccessCallback(response) {
//handle success code here
}
You may prefer to make this a bit cleaner by wrapping up the closure in another method:
PageMethods.SaveData(
JSON.stringify(basicInfo), SaveSuccessCallback(10), SaveFailedCallback);
function SaveSuccessCallback(a) {
return function(response) {
//handle success code here
ShowEdit(a);
};
}