ngRoute adds trailing slash to some url but not all - angularjs

Question

When user go to either localhost:8888 or localhost:8888/, my angular application change to url to http://localhost:8888/#!/
This is my angular app in home page:
app.config(['$locationProvider', '$routeProvider',
function($locationProvider, $routeProvider) {
$locationProvider.hashPrefix('!');
$routeProvider
.when('/', {
redirectTo: ((window.status === 'signup') ? '/signup' : '/signin')
})
.when("/signin", {
templateUrl: "/public/user/signin.html",
controller: "signinController"
})
.when('/signup', {
templateUrl: "/public/user/signup.html",
controller: "signupController"
})
.otherwise({
redirectTo: '/'
});
}
])
if user go to http://localhost:8888/auth, angular redirects to http://localhost:8888/auth#!/signin,
only if user go to http://localhost:8888/auth/, angular redirects to http://localhost:8888/auth/#!/signin
app.config(['$locationProvider', '$routeProvider',
function($locationProvider, $routeProvider) {
$locationProvider.hashPrefix('!');
$routeProvider
.when('/', {
redirectTo: ((window.status === 'signup') ? '/signup' : '/signin')
})
.when("/signin", {
templateUrl: "/public/user/signin.html",
controller: "signinController"
})
.when('/signup', {
templateUrl: "/public/user/signup.html",
controller: "signupController"
})
.otherwise({
redirectTo: '/'
});
}
])
the angular official website and most of the books recommend using xxx/#!/xxx format. How can I do that?
Edit: I understand that I can add the trailing slash from the server side. but user can directly type xxx.com/auth.
Why is xxx.com working but not xxx.com/auth?
Edit2: the tutorial sample project works for all urls. we have pretty much the same implementation
http://angular.github.io/angular-phonecat/step-8/app/#/phones
try to enter http://angular.github.io/angular-phonecat/step-8/app (without the trailing slash
Tutorial link: https://docs.angularjs.org/tutorial/step_07

Answer 1

This is something you should fix on your back-end by adding a 301 redirect from /auth to /auth/.
Nginx:
rewrite ^/auth$ /auth/ permanent;
Apache:
Redirect "/auth" "/auth/" [R=301]