javascript - Angularjs md-select not set placeholder -

following code of <md-select/> not showing placeholder when added placeholder="select type"

<md-select ng-model="ctrl.number" ng-model-options="{trackby: '$value.id'}" placeholder="select type">     <md-option ng-repeat="n in ctrl.numbers" ng-value="n">       {{ n.value }}     </md-option> </md-select> 

but work below code

  <md-select ng-model="ctrl.number"  placeholder="select type">     <md-select-label>{{ ctrl.number.value }}</md-select-label>     <md-option ng-repeat="n in ctrl.numbers" ng-value="n">{{ n.value }}</md-option>   </md-select> 

js code

(function () {   'use strict';   angular       .module('myapp')       .controller('appctrl', function() {         var _this = this;          _this.number = {id: 2, value: "two"};         _this.numbers = [           {id: 0, value: "zero"},           {id: 1, value: "one"},           {id: 2, value: "two"},           {id: 3, value: "three"},           {id: 4, value: "four"},           {id: 5, value: "five"}         ];       }); })(); 

can please let me know wrong first , how set placeholder in case ?

<md-select ng-model="ctrl.number" ng-model-options="{trackby: '$value.id'}"  placeholder="select type"> <md-option value="">select type</md-option> <md-option ng-repeat="n in ctrl.numbers" ng-value="n">   {{ n.value }} </md-option>` </md-select> 

your code works fine. placeholder displays when not selected. should add "md-option value="" unselect.