API 意为 Application Programming Interface（应用程序编程接口）。
通用的 API 函数：
| API | 描述 |
|---|---|
| angular.lowercase() | 转换字符串为小写 |
| angular.uppercase() | 转换字符串为大写 |
| angular.isString() | 判断给定的对象是否为字符串，如果是返回 true。 |
| angular.isNumber() | 判断给定的对象是否为数字，如果是返回 true。 |
angular.lowercase()实例：
<div ng-app="myApp" ng-controller="myCtrl">
<p>{{ x1 }}</p>
<p>{{ x2 }}</p>
</div>
<script>
var app = angular.module(‘myApp‘, []);
app.controller(‘myCtrl‘, function($scope) {
$scope.x1 = "JOHN";
$scope.x2 = angular.lowercase($scope.x1);
});
</script>
angular.uppercase()实例：
<div ng-app="myApp" ng-controller="myCtrl">
<p>{{ x1 }}</p>
<p>{{ x2 }}</p>
</div>
<script>
var app = angular.module(‘myApp‘, []);
app.controller(‘myCtrl‘, function($scope) {
$scope.x1 = "John";
$scope.x2 = angular.uppercase($scope.x1);
});
</script>
angular.isString()实例：
<div ng-app="myApp" ng-controller="myCtrl">
<p>{{ x1 }}</p>
<p>{{ x2 }}</p>
</div>
<script>
var app = angular.module(‘myApp‘, []);
app.controller(‘myCtrl‘, function($scope) {
$scope.x1 = "JOHN";
$scope.x2 = angular.isString($scope.x1);
});
</script>
angular.isNumber()实例：
<div ng-app="myApp" ng-controller="myCtrl">
<p>{{ x1 }}</p>
<p>{{ x2 }}</p>
</div>
<script>
var app = angular.module(‘myApp‘, []);
app.controller(‘myCtrl‘, function($scope) {
$scope.x1 = "JOHN";
$scope.x2 = angular.isNumber($scope.x1);
});
</script>
原文：http://www.cnblogs.com/jiangtengteng/p/5992472.html