This demo shows how you can use maps in GeoJSON format on JavaScript Maps.
As it says on their website, GeoJSON is a format for encoding a variety of geographic data structures. That includes map information.
We are using two additional libraries here: jQuery to AJAX-load the JSON data for our map, and geojson2svg library. The latter does most of the magic - as its name implies, it converts the data in GeoJSON format to SVG - what is used to display the maps.