<!doctype html>
<html lang="en">
<head>
<title>JavaScript Editor</title>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta charset="utf-8">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
</head>
<body>
<div id="page"></div>
<textarea id="editor"></textarea>
<script>
$("#editor").val(localStorage.html) // editor <- var
$("#page").html(localStorage.html) // page <- var
function sync(){
localStorage.html=$('#editor').val(); // var <- editor
$("#page").html(localStorage.html) // page <- var
}
</script>
<button onclick="sync()">sync</button>
</body>
</html>