<select  class="ui fluid multiple selection dropdown" id="multi-select">
                        <option *ngFor="let objectif of listObjectif" [value]="objectif.id_obj">{{objectif.des_obj}}
                        </option>
                    </select>

this.selectedObjectif = <[any]>[];
    this.selectedObjectif = $('select#multi-select').dropdown('get value');

(2) ["1", Array(4)]
0:"1"
1:(4) ["1", "8", "3", "16"]
length:2