Handle empty geometries as undefined @turf/buffer

packages/turf-buffer/README.md

@@ -6,8 +6,8 @@ Calculates a buffer for input features for a given radius. Units supported are m
 
 **Parameters**
 
--   `feature` **([FeatureCollection](http://geojson.org/geojson-spec.html#feature-collection-objects) \| [Feature](http://geojson.org/geojson-spec.html#feature-objects)<any>)** input to be buffered
--   `radius` **[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)** distance to draw the buffer
+-   `feature` **([FeatureCollection](http://geojson.org/geojson-spec.html#feature-collection-objects) \| [Geometry](http://geojson.org/geojson-spec.html#geometry) \| [Feature](http://geojson.org/geojson-spec.html#feature-objects)<any>)** input to be buffered
+-   `radius` **[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)** distance to draw the buffer (negative values are allowed)
 -   `units` **\[[string](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)]** any of the options supported by turf units (optional, default `kilometers`)
 -   `steps` **\[[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)]** number of steps (optional, default `64`)
 
@@ -28,7 +28,7 @@ var buffered = turf.buffer(point, 500, 'miles');
 var addToMap = [point, buffered]
 ```
 
-Returns **([FeatureCollection](http://geojson.org/geojson-spec.html#feature-collection-objects) \| [Feature](http://geojson.org/geojson-spec.html#feature-objects)<([Polygon](http://geojson.org/geojson-spec.html#polygon) \| [MultiPolygon](http://geojson.org/geojson-spec.html#multipolygon))>)** buffered features
+Returns **([FeatureCollection](http://geojson.org/geojson-spec.html#feature-collection-objects) \| [Feature](http://geojson.org/geojson-spec.html#feature-objects)<([Polygon](http://geojson.org/geojson-spec.html#polygon) \| [MultiPolygon](http://geojson.org/geojson-spec.html#multipolygon))> | [undefined](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined))** buffered features
 
 <!-- This file is automatically generated. Please don't edit it directly:
 if you find an error, edit the source file (likely index.js), and re-run

packages/turf-buffer/index.d.ts

@@ -18,12 +18,12 @@ interface Buffer {
     /**
      * http://turfjs.org/docs/#buffer
      */
-    <Geom extends Point|LineString|Polygon>(feature: Feature<Geom>|Geom, radius?: number, unit?: Units, steps?: number): Feature<Polygon>;
-    <Geom extends MultiPoint|MultiLineString|MultiPolygon>(feature: Feature<Geom>|Geom, radius?: number, unit?: Units, steps?: number): Feature<MultiPolygon>;
+    <Geom extends Point|LineString|Polygon>(feature: Feature<Geom>|Geom, radius?: number, unit?: Units, steps?: number): Feature<Polygon> | undefined;
+    <Geom extends MultiPoint|MultiLineString|MultiPolygon>(feature: Feature<Geom>|Geom, radius?: number, unit?: Units, steps?: number): Feature<MultiPolygon> | undefined;
     <Geom extends Point|LineString|Polygon>(feature: FeatureCollection<Geom>, radius?: number, unit?: Units, steps?: number): FeatureCollection<Polygon>;
     <Geom extends MultiPoint|MultiLineString|MultiPolygon>(feature: FeatureCollection<Geom>, radius?: number, unit?: Units, steps?: number): FeatureCollection<MultiPolygon>;
     (feature: FeatureCollection<any>|FeatureGeometryCollection|GeometryCollection, radius?: number, unit?: Units, steps?: number): FeatureCollection<Polygon|MultiPolygon>;
-    (feature: Feature<any>|GeometryObject, radius?: number, unit?: Units, steps?: number): Feature<Polygon|MultiPolygon>;
+    (feature: Feature<any>|GeometryObject, radius?: number, unit?: Units, steps?: number): Feature<Polygon|MultiPolygon> | undefined;
 }
 declare const buffer: Buffer;
 declare namespace buffer {}

packages/turf-buffer/index.js

@@ -15,11 +15,11 @@ var distanceToRadians = helpers.distanceToRadians;
  * Calculates a buffer for input features for a given radius. Units supported are miles, kilometers, and degrees.
  *
  * @name buffer
- * @param {FeatureCollection|Feature<any>} feature input to be buffered
- * @param {number} radius distance to draw the buffer
+ * @param {FeatureCollection|Geometry|Feature<any>} feature input to be buffered
+ * @param {number} radius distance to draw the buffer (negative values are allowed)
  * @param {string} [units=kilometers] any of the options supported by turf units
  * @param {number} [steps=64] number of steps
- * @return {FeatureCollection|Feature<Polygon|MultiPolygon>} buffered features
+ * @return {FeatureCollection|Feature<Polygon|MultiPolygon>|undefined} buffered features
  * @example
  * var point = {
  *   "type": "Feature",
@@ -53,14 +53,18 @@ module.exports = function (geojson, radius, units, steps) {
     switch (geojson.type) {
     case 'GeometryCollection':
         geomEach(geojson, function (geometry) {
-            results.push(buffer(geometry, radius, units, steps));
+            var buffered = buffer(geometry, radius, units, steps);
+            if (buffered) results.push(buffered);
         });
         return featureCollection(results);
     case 'FeatureCollection':
         featureEach(geojson, function (feature) {
-            featureEach(buffer(feature, radius, units, steps), function (buffered) {
-                results.push(buffered);
-            });
+            var multiBuffered = buffer(feature, radius, units, steps);
+            if (multiBuffered) {
+                featureEach(multiBuffered, function (buffered) {
+                    if (buffered) results.push(buffered);
+                });
+            }
         });
         return featureCollection(results);
     }
@@ -88,7 +92,8 @@ function buffer(geojson, radius, units, steps) {
     case 'GeometryCollection':
         var results = [];
         geomEach(geojson, function (geometry) {
-            results.push(buffer(geometry, radius, units, steps));
+            var buffered = buffer(geometry, radius, units, steps);
+            if (buffered) results.push(buffered);
         });
         return featureCollection(results);
     }
@@ -108,11 +113,25 @@ function buffer(geojson, radius, units, steps) {
     var writer = new jsts.io.GeoJSONWriter();
     buffered = writer.write(buffered);
 
+    // Detect if empty geometries
+    if (coordsIsNaN(buffered.coordinates)) return undefined;
+
     // Unproject coordinates (convert to Degrees)
     buffered.coordinates = unprojectCoords(buffered.coordinates, projection);
     return feature(buffered, properties);
 }
 
+/**
+ * Coordinates isNaN
+ *
+ * @private
+ * @param {Array<any>} coords GeoJSON Coordinates
+ * @returns {Boolean} if NaN exists
+ */
+function coordsIsNaN(coords) {
+    if (Array.isArray(coords[0])) return coordsIsNaN(coords[0]);
+    return isNaN(coords[0]);
+}
 
 /**
  * Project coordinates to projection

packages/turf-buffer/test.js

@@ -79,6 +79,16 @@ test('turf-buffer - Prevent Input Mutation', t => {
     t.end();
 });
 
+// /~https://github.com/Turfjs/turf/issues/745
+// /~https://github.com/Turfjs/turf/pull/736#issuecomment-301937747
+test('turf-buffer - morphological closing', t => {
+    const poly = polygon([[[11, 0], [22, 4], [31, 0], [31, 11], [21, 15], [11, 11], [11, 0]]]);
+
+    t.equal(buffer(poly, -500, 'miles'), undefined, 'empty geometry should be undefined');
+    t.deepEqual(buffer(featureCollection([poly]), -500, 'miles'), featureCollection([]), 'empty geometries should be an empty FeatureCollection');
+    t.end();
+});
+
 function colorize(feature, color = '#F00') {
     if (feature.properties) {
         feature.properties.stroke = color;