This version of Mapshaper is maintained for readers of Implications of Weighting Metrics for Line Generalisation with Visvalingam’s Algorithm (Visvalingam and Whelan, 2014). The latest version (available here) uses a different function for weighted Visvalingam simplification than the function discussed in the paper.
There are several options for those who would like to help improve Mapshaper. You can:
Drag a Shapefile .shp, GeoJSON or TopoJSON file onto this window, or select a file from a local folder.