turf.union
What is turf.union?
turf.union merges two or more polygons into a single Polygon or MultiPolygon, combining overlapping or adjacent geometry. It is the GeoJSON equivalent of ST_Union in PostGIS and uses the robust Martinez-Rueda algorithm via polygon-clipping.
turf.union(featureCollection) → Feature<Polygon | MultiPolygon> | nullRecent versions accept a FeatureCollection<Polygon>; legacy versions took a variadic list of polygon arguments. Confirm the signature in your installed version.
When would you use turf.union?
Use turf.union to combine geofences into a single coverage area, merge drawn selections as a user adds to a shape, or collapse a multi-polygon search region into a single boundary. It is also the go-to operation for "total footprint" calculations — e.g., combining building outlines to compute total rooftop area.
Unlike turf.dissolve, turf.union does not require inputs to share edges exactly and handles overlapping polygons gracefully.
1import union from '@turf/union';
2import { featureCollection } from '@turf/helpers';FAQs
What is the difference between union and dissolve?
turf.union handles arbitrary polygons — overlapping, disjoint, or adjacent — and always produces a single merged output. turf.dissolve expects a tiled input with shared edges and can filter by an attribute. Use dissolve for attribute-based grouping; union for free-form merging.
How do I install just union?
npm install @turf/union. It depends on @turf/helpers and polygon-clipping.
Can it merge more than two polygons?
Yes — pass any number of polygons in the FeatureCollection. Internally the algorithm unions them pairwise.
What if inputs are disjoint?
The result is a MultiPolygon containing all disjoint parts. You can split with turf.flatten if you need individual Polygon features.
