爲什麼ng-class甚至需要額外的引號

我正在angularJS上的代碼很好，但我被困在一個地方，我找不到爲什麼在ng-class-even指令中需要額外的引號。爲什麼ng-class甚至需要額外的引號

在這一行<tr ng-repeat="x in records" ng-class-even='"striped"'>爲什麼條紋是這樣寫的"'striped'"，而不是這個"striped"

.striped { 
 
    color:white; 
 
    background-color:black; 
 
}

<body ng-app="myApp"> 
 
<script src="http://ajax.googleapis.com/ajax/libs/angularjs/1.4.8/angular.min.js"></script> 
 

 
<table ng-controller="myCtrl"> 
 
<tr ng-repeat="x in records" ng-class-even="'striped'"> 
 
    <td>{{x.Name}}</td> 
 
    <td>{{x.Country}}</td> 
 
</tr> 
 
</table> 
 

 
<script> 
 
var app = angular.module("myApp", []); 
 
app.controller("myCtrl", function($scope) { 
 
    $scope.records = [ 
 
    { 
 
     "Name" : "Alfreds Futterkiste", 
 
     "Country" : "Germany" 
 
    }, 
 
    { 
 
     "Name" : "Berglunds snabbköp", 
 
     "Country" : "Sweden" 
 
    }, 
 
    { 
 
     "Name" : "Centro comercial Moctezuma", 
 
     "Country" : "Mexico" 
 
    }, 
 
    { 
 
     "Name" : "Ernst Handel", 
 
     "Country" : "Austria" 
 
    } 
 
    ] 
 
}); 
 
</script> 
 

 
</body>

來源

2016-06-09 Gaurav Aggarwal