Javascript array push problems

Question

I want to store dates taken from a GET request into the array vm.dates. For doing this I use the function push(), later, I want to check whether the date is contained in the array or not. The problem is that when I exit the $http function, I am not able to access the data contained in vm.dates.

I performed the following tests: when I print the array, I read all the dates stored, but when I check the length it returns 0 and if I iterate on it or I want to access an element at index x, it returns undefined. Why this problem? I looked through various questions on Stack Overflow and internet, but no answer yet.

Attached is the portion of code involved.

.controller('PlansController', function($http, $log) {

  var vm = this;

  vm.events = [];
  vm.dates = [];

  var req = {
    method: 'GET',
    url: '***',
    headers: {
      'Content-Type': "application/json"
    }
  }

  $http(req)
    .then(function(response) {

      var workouts = response.data.workouts;
      var headers = response.headers();
      for (var num in workouts) {
        var duration = Math.round(workouts[num].duration / 60, 0);
        var startingDate = new Date(workouts[num].date);
        vm.events.push({
          title: workouts[num].muscle_group + "\n" + duration + " min",
          start: startingDate,
          allDay: true
        });

        vm.dates.push(startingDate.valueOf());

      }
    });

  /* config object */
  vm.uiConfig = {
    dayRender: function(date, cell) {

      function elementInArray(el, arr) {
        for (var ind in arr) {
          var intInd = parseInt(arr[ind], 10)
          var intEl = parseInt(el, 10)
          if (intInd == intEl) {
            return true;
          }
        }
        return false;
      }

      var today = new Date();
      var dateRendered = date._d;
      if (cell.hasClass('fc-other-month')) {
        cell.css("background-color", "#fff");
        if (elementInArray(dateRendered.valueOf(), vm.dates)) {
          cell.css("background-color", "#000");
        }
      } else if (cell.hasClass('fc-today')) {
        cell.css("background-color", "#fff");
        if (elementInArray(dateRendered.valueOf(), vm.dates)) {
          cell.css("background-color", "#000");
        }
      } else {
        cell.css("background-color", "#fff");
        //$log.info(typeof(vm.dates[1]));
        if (elementInArray(dateRendered.valueOf(), vm.dates)) {
          cell.css("background-color", "#000");
        }
      }

    }
  }
};

/* Event sources array */
vm.eventSources = [vm.events];
});