Firebase web logout() not redirect to index page

I am using firebase auth for my web project. I use the code below to sign out but it keeps redirect me to homepage instead of index page.

function logout(){

firebase.auth().signOut().then(function() {
  // Sign-out successful.
  document.location.href = 'index.html';
}).catch(function(error) {
  // An error happened.
});

}

I have tried this code but its not redirect to anywhere like the button is not function at all.

firebase.auth().signOut();