cats.json

{% header "Content-Type: application/json" %}
[
{% nav category in craft.categories.group('categories') %}
{
"name": "{{ category.title }}",
"url": "{{ category.url }}",
"loop": "{{ loop.index }}",
{% ifchildren %},
"children": [
{% children %}
]
{% endifchildren %}
}
{% if not loop.last %},{% endif %}
{% endnav %}
]