[skrudjmakdak]

можно сделать функцию обратного вызова:

....
myobj.prototype.loadJSON = function(callback) 
{
	var that = this;
	$.getJSON('json/test.json', function(data) {
		that.loadedJSON = data;
		that.prepareJSON();
if (typeof callback == 'function')
	callback(that, true);
	}).fail(function() {
		console.log("Ошибка JSON");
if (typeof callback == 'function')
	callback(that, false);
	});

	return this.loadedJSON;
}
......
x = new ft();
x.loadJSON(function (obj, success) {
//actions
});