The simplest version of what Vincent Scheib said:
index.html
...
<div id="screen1" style="display:block">
...
</div>
<div id="screen2" style="display:none">
...
</div>
main.js
...
// A navigational event happens:
document.getElementById("screen1").style.display = "none";
document.getElementById("screen2").style.display = "block";
...