Typescript - call child method from parent class
<p>I am trying to call the child method from the abstract parent class in typescript. But i am getting this error:</p>

<blockquote>
  <p>Uncaught TypeError: this.activateMultiselect is not a function</p>
</blockquote>

<p>How would I implement this without error?</p>

<p>Here is the code:</p>

<pre><code>interface iGetAreas {
    _areasList: Array&lt;string&gt;;
    _areas: KnockoutObservableArray&lt;string&gt;;
    _selectedArea: KnockoutObservable&lt;string&gt;;
    getAreas(geonameId: string);
    activateMultiselect();
}

abstract class AreaGetter implements iGetAreas {
    _areasList = [];
    _areas = ko.observableArray([]);
    _selectedArea = ko.observable('');

    abstract activateMultiselect();

    getAreas(geonameId){
        var self = this;
        self._areasList = [];
        $.ajax({
            url: `http://api.geonames.org/children?geonameId=${geonameId}&amp;username=elion`
        }).then(function(allAreasXML) {
            var allAreasJSON = xml2json(allAreasXML);
            var allAreas = JSON.parse(allAreasJSON);
            if(allAreas.geonames.length) {
                for (var index = 1; index &lt; allAreas.geonames.length - 1; index++) {
                self._areasList.push(allAreas.geonames[index].geoname);
                }
            } else {
                if(allAreas.geonames) {
                    self._areasList.push(allAreas.geonames.geoname);
                }
            }
            self._areas(self._areasList);
            this.activateMultiselect();
        });
    }
}

class RegionGetter extends AreaGetter {
    activateMultiselect() {
        $("#region-select").multiselect({
            buttonWidth: '100%',
            buttonContainer: '&lt;div style="height: 64px;" /&gt;',
            buttonClass: 'none',
            onChange: function(option, checked, select) {
                alert('Changed region option ' + $(option).val() + '.');
            }
        });
    }
}

class TownGetter extends AreaGetter {
    activateMultiselect() {
        $("#town-select").multiselect({
            buttonWidth: '100%',
            buttonContainer: '&lt;div style="height: 64px;" /&gt;',
            buttonClass: 'none',
            onChange: function(option, checked, select) {
                alert('Changed town option ' + $(option).val() + '.');
            }
        });
    }
}
</code></pre>