Check LocalStorage Constantly - angularjs

Question

Is there a way to check the client localstorage at all times with angularjs? The reason I want to do this is because I have set a JSON Web Token in the local storage when the user logs in. Furthermore, I am using
$rootScope.$on('$routeChangeStart', function() {});
to check the token. Once it is found to have expired, the user is logged out. However, with this method, the user has to make an API request in order for the check to be reinitialized. Is there a way to observe the token that is stored inside of the localStorage and as soon as it expires, then log the user out without the need for an API request?

Answer 1

Was able to use #IAMDranged 's suggestion:
setInterval(function() {
var token = $window.localStorage.getItem('token');
self.parseJwt = function(token) {
var base64Url = token.split('.')[1];
var base64 = base64Url.replace('-', '+').replace('_', '/');
return JSON.parse($window.atob(base64));
}
var expireTime = self.parseJwt(token);
var timeStamp = Math.floor(Date.now() / 1000);
var timeCheck = expireTime.exp - timeStamp;
//Set to expire after 15 seconds
if (timeCheck < 86385) {
console.log('time expired');
} else {
console.log('time not expired');
}
}, 3000);

Answer 2

When you create/get the token you don't know after how much time it will be expires? because if you know you can save the expiration time/date in the localStorage and when the page is loaded or the token changed (the user login or logout) use setTimeout to logout the user when the token should be expired.
If you don't know the expiration time (what i think is the case) the best think you can do is to send the token to the server on every request and make the server check the token on every request (consider using cookie instead of localStorage), if the token is ok let the server do the work, otherwise make the server return 403 http error and in the client side use $httpInterceptor to check if the server return status 403 and logout the user (instead of doing request to check the token on every state change).
Edit: Add Code example (for the first solution). You should call this function when the token changed too.
angular.module('...')
.run(['$timeout', function ($timeout) {
var _timeout;
function deleteTokenTimeout() {
if (_timeout) {
_timeout.cancel();
}
_timeout = $timeout(function () {
// remove the token
}, new Date(localStorage.getItem('expiration')) - new Date());
}
deleteTokenTimeout();
}]);