It should be parentNode , not just parent :
alert(obj.target.parentNode.id);
Also attributes is redundant, as you can directly access the id property.
But note that since you have ngRepeat , it will create invalid markup as identifiers will be duplicated. You probably want to fix this too, maybe this is the case or use the classes:
<div id="d8{{$index}}" class="menutitles ng-scope" ng-repeat="title in list"> <div class="right" ng-click="showsubmenu()">+</div> <div class="left" ng-click="showsubmenu()">Unit 9</div> </div>
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