L.TopoJSON

var layer = new L.TopoJSON(topojson_data).addTo(map);

adds a TopoJSON layer to a leaflet map. The usage is identical to L.GeoJSON().

Typical usage is below:

$.get("topojson-file.json").done(function (topojson_data) {
  var map = L.map("map-id");
  var layer = new L.TopoJSON(topojson_data).addTo(map);
  map.fitBounds(layer.getBounds());
});