ViewModel class in Typescript

I am trying to refactor a ViewModel class because it has two methods that are really similar called updateRegions and updateTowns and even another one may end up being made called updateCountries. I basically copy-paste the method and change the variable names. How can this be refactored with maybe an abstract class that has an abstract updateArea function?

class SearchFilterViewModel {
        _regionsList = [];
        _townsList = [];
        _regions = ko.observableArray();
        _towns = ko.observableArray();
        selectedRegion: KnockoutObservable<string> = ko.observable('');
        selectedTown: KnockoutObservable<string> = ko.observable('');
        selectedCategories: KnockoutObservableArray<string> = ko.observableArray([]);

        constructor() {
            this.updateRegions("2186224");
            this.selectedRegion.subscribe(this.updateTowns.bind(this));
        }

        updateRegions(geonameId) {
            var self = this;
            self._regionsList = [];
            $.ajax({
                url: `http://api.geonames.org/children?geonameId=${geonameId}&username=elion`
            }).then(function(allRegionsXML) {
                var allRegionsJSON = xml2json(allRegionsXML);
                var allRegions = JSON.parse(allRegionsJSON);
                if(allRegions.geonames.length) {
                    for (var index = 1; index < allRegions.geonames.length - 1; index++) {
                    self._regionsList.push(allRegions.geonames[index].geoname);
                    }
                } else {
                    if(allRegions.geonames) {
                        self._regionsList.push(allRegions.geonames.geoname);
                    }
                }
                self._regions(self._regionsList);
            });
        }

        updateTowns(geonameId) {
            var self = this;
            self._townsList = [];
            $.ajax({
                url: `http://api.geonames.org/children?geonameId=${geonameId}&username=elion`
            }).then(function(allTownsXML) {
                var allTownsJSON = xml2json(allTownsXML);
                var allTowns = JSON.parse(allTownsJSON);
                if(allTowns.geonames.length) {
                    for (var index = 1; index < allTowns.geonames.length - 1; index++) {
                    self._townsList.push(allTowns.geonames[index].geoname);
                    }
                } else {
                    if(allTowns.geonames) {
                        self._townsList.push(allTowns.geonames.geoname);
                    }
                }
                self._towns(self._townsList);
            });
        }
    }