you can use the local storage like this :
var fn = document.getElementById("firstname").value;
localStorage.setItem("firstname", fn);
var ln = document.getElementById("lastname").value;
localStorage.setItem("lastname", ln);
var em = document.getElementById("email").value;
localStorage.setItem("email", em);
thus the clients browser will have these items set in their local storage. Now if a user visits the website afterwards. you can check for the value of localStorage and find the items of your need.
Suppose on users' next visit you want to send him a greet message ( he has not logged in ofcourse ) you can use a script like this below:
var name = localStorage.getItem("firstname");
alert("Hello"+name);