ee.ImageCollection.union

Merges all geometries in a given collection into one and returns a collection containing a single feature with only an ID of 'union_result' and a geometry.
UsageReturns
ImageCollection.union(maxError)FeatureCollection
ArgumentTypeDetails
this: collectionFeatureCollectionThe collection being merged.
maxErrorErrorMargin, default: nullThe maximum error allowed when performing any necessary reprojections. If not specified, defaults to the error margin requested from the output.