角$ scope具有未定义的push（）

Question

Here is my HTML:

<input ng-blur="CheckUser()" name="username" ng-model="RegisterFormData.username"
                                           class="form-control"/>{{ a.check_username_result }}

The controller and ng-app are assigned, since every part of the application is working and the only missing part is that I cannot send back data after $http request to the page, it says property of push() is undefined

Now, here is my controller and CheckUser() is the event which sends a certain data back to the page after some operation, though all operations are done properly, except the push() which errors as undefined:

LoginApp.controller("RegisterController", function($scope, $http, registerService){
    var username  = null;
    var password = null;

    RegisterData = $scope.RegisterFormData;

    $scope.CheckUser = function(RegisterData){

        username = $scope.RegisterFormData.username;
        console.log(username);
        usernameResult = registerService.CheckUser(username);


        if( usernameResult == "exists")
            $scope.a.push({check_username_result : "Username already exists, Choose another."});
        else if( usernameResult == 'fresh')
            $scope.a.push({check_username_result : "Username available!"});
        else
            $scope.a.push({check_username_result : "Error. try again."});
    }; // end of user_check()

    $scope.registerSubmit  = function(){
        username = $scope.RegisterFormData.username;
        password = $scope.RegisterFormData.password;


    };
});

Answer 1

Just declare $scope.a as an array, then you can push to it.

$scope.a = [];
$scope.CheckUser = function(RegisterData){
// ...

Answer 2

You cannot call functions on an object before it it defined. You can do

$scope.a=[];

And then

$scope.a.push({check_username_result : "Username already exists, Choose another."});