Leaflet TrackStats is a plugin for Leaflet to compute altitude/slope statistics on paths.
L.TrackStats extends Leaflet's Polyline objects to include methods to compute the stats. It also integrates nicely with Leaflet TrackDrawer.
This plugin only supports MapQuest and GeoPortail as data providers (you will need your own API keys by the way), but it can easily be extended for new data providers by implementing the IFetcher interface.
A real-case usage of this plugin is for map2gpx.
See doc.