track by

ng-repeat指令中使用track by子语句解决重复数据遍历的错误

<li ng-repat="x in [2, 2]" ng-bind="x"></li>
是错误的，认为不能出现重复值(id)

<li ng-repat="x in [2, 2] track by $id($index)" ng-bind="x"></li>