timkit,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<ul id="tree">
<li data-id="0">Автомобили
<ul>
<li data-id="1">Japan
<ul>
<li data-id="2">Toyota
<ul>
<li data-id="3">Camry</li>
<li data-id="4">Avensis</li>
</ul>
</li>
<li data-id="5">Mazda</li>
</ul>
</li>
</ul>
</li>
</ul>
<script>
function h(b, c) {
b = b.children;
for (var d = b.length, a; a = b[--d];) "li" == a.tagName.toLowerCase() ? (c['li '+a.dataset.id] = {},
c['li '+a.dataset.id][a.firstChild.data] = h(a, {})) : c.ul = h(a, {});
return c
};
var data = h(document.getElementById('tree'), {})
document.write(JSON.stringify(data))
</script>
</body>
</html>