JSFiddle - React, Tailwind, and code Playground

HTML

<div ng-app="app">
    <div ng-controller="testCtrl">
        <select data-ng-model="first"  data-ng-options="item.name for item in selectContent" required="required">
          <option value="" style="display: none;">-- select --</option>
        </select>
        {{first.value}}
        <hr/>
        <input type="button" value="reset dropdown" data-ng-click="resetDropDown()"/>
    </div>
</div>

JavaScript

angular.module('app', []).controller('testCtrl', function ($scope) {
    $scope.selectContent = [
        {
            name: "first",
            value: "1"
        },
        {
            name: "second",
            value: "2"
        }
    ];
    $scope.first = $scope.selectContent[0].value;
    $scope.resetDropDown = function() {
        if(angular.isDefined($scope.first)){
            delete $scope.first;
        }
    }
});