Drag and drop, or use the upload button to put a geojson file, topojson or a zipped shapefile onto a map. (geojson and topojson can also be zipped)
The shapefile is parsed to GeoJSON using shapefile-js, which I also wrote, which uses DataView and array buffers to parse this mixed endian file. There are also promises used in here, kids these days like promises right?
If you need a file to show here is a zipped shapefile of the world
There is a large amount of geojson in these repos