mapbox/geojson-merge
Merge multiple GeoJSON files into one FeatureCollection. $ npm install --save @mapbox/geojson-merge Merge a series of GeoJSON objects into one FeatureCollection containing all features in all files. The objects can be any valid GeoJSON root object, including FeatureCollection, Feature, and Geometry types. Parameters Examples Returns Object a geojson FeatureCollection.
https://github.com/mapbox/geojson-merge