Why I get this error: Uncaught TypeError: Cannot read property 'serviceId' of undefined

I can display value of serviceId to console but when I try to compare it in if(), I got this error. Can you tell me why does this happend and how to solve it, please?

        var servicesCompare1 = [];
        var servicesCompare2 = [];
        $.getJSON("http://localhost:55972/api/status", function (data) {
            self.services(data.services);
            self.lastCheck = data.lastCheck;
            servicesCompare1 = (data.services);
        });

        function DashboardRefresh() {
            self.servicesRefresh = ko.observable([]);
            $.getJSON("http://localhost:55972/api/status", function (data) {
                servicesCompare2 = (data.services);
                self.servicesRefresh(data.services);                
            });
          if (servicesCompare2.length > servicesCompare1.length) {
              for (i = 0; i < (servicesCompare2.length-1); i++) {                   
                  console.log(servicesCompare2[i].serviceId);
                  if (servicesCompare1[i].serviceId !== servicesCompare2[i].serviceId) {
                      self.services.push(servicesCompare2[i]);

                  }
              }
          } 
        }
        setInterval(DashboardRefresh, 5000);

Console output: 47 48 49 ... other Ids

Uncaught TypeError: Cannot read property 'serviceId' of undefined at DashboardRefresh (NetworkStatus.html:66)