Where am I going wrong with this keyword while passing it as a parameter in the ng-click function call in angular 1.5x?

I am trying to pass data to an API for the user e-mail subscription status as "Y"/"N". In the controller code " console.log(a.checked)" is undefined.But in regular javascript onclick event for the input element type="checkbox" has the "this.checked" to respond correspondingly as true or false.Why is it not happening here in angularJs.

// this is my html code with angular directive ng-click

 <label for="mail_sub">E-mail subscription</label>
                        <input id="mail_sub" type="checkbox" name=checkbox ng-click="mailsubscription(this)">

// this part of code is of the controller

    .controller("registerCtrl", function($scope,$state,userProcess){
        $scope.mailsubscription = function(a){
            console.log(a);
            console.log(a.checked);// console output is: "undefined"
            signupinfo = {};
            if(a.checked){
                signupinfo.u_mail_subscription = 'Y';
            }else{
                signupinfo.u_mail_subscription = 'N';
            }
            console.log(signupinfo);

        };
        /*$scope.registerProcess = function(signupinfo){
            console.log(signupinfo);
            userProcess.signup(signupinfo).sucess(function(response){

                if(response.status){


                }
            })

        };*/
    });