如何在angularjs中调用粗体函数？

Question

我想更改Textarea中的字体属性（粗体或斜体）。下面我放置了我的代码供您参考。请让我知道如何在我的脚本中调用粗体属性。

HTML：

<div ng-app='myNoteApp' ng-controller='noteController'>
    <p>
        <textarea ng-model='message' cols='50' rows='10'></textarea>
    </p>
    <p>
        <button ng-click='save()'>Save</button>
        <button ng-click='clear()'>Clear</button>
        <button ng-click='bold()'>Bold</button>
    </p>
    <p>Numbers of characters left: <span ng-bind='left()'></span></p>
</div>

myNoteApp JS：

var app = angular.module('myNoteApp', []);

myNoteCtrl JS：

app.controller('noteController', function($scope){
    $scope.message='';
    $scope.save=function(){alert('Saved!');};
    $scope.clear=function(){$scope.message='';};
    $scope.left=function(){return 300-$scope.message.length;};
    $scope.bold=function(){return $scope.bold;};
});