diff --git a/index.js b/index.js
index 4aba334..0e310fb 100644
--- a/index.js
+++ b/index.js
@@ -1,40 +1,51 @@
-var wgs84 = require('wgs84');
+'use strict';
+
+const wgs84 = require('wgs84');
 
 module.exports.geometry = geometry;
 module.exports.ring = ringArea;
-
 function geometry(_) {
-    var area = 0, i;
-    switch (_.type) {
-        case 'Polygon':
-            return polygonArea(_.coordinates);
-        case 'MultiPolygon':
-            for (i = 0; i < _.coordinates.length; i++) {
-                area += polygonArea(_.coordinates[i]);
-            }
-            return area;
-        case 'Point':
-        case 'MultiPoint':
-        case 'LineString':
-        case 'MultiLineString':
-            return 0;
-        case 'GeometryCollection':
-            for (i = 0; i < _.geometries.length; i++) {
-                area += geometry(_.geometries[i]);
-            }
-            return area;
-    }
+  let area = 0,
+    i;
+  switch (_.type) {
+    case 'Polygon':
+      return polygonArea(_.coordinates);
+    case 'MultiPolygon':
+      for (i = 0; i < _.coordinates.length; i++) {
+        area += polygonArea(_.coordinates[i]);
+      }
+      return area;
+    case 'Point':
+    case 'MultiPoint':
+    case 'LineString':
+    case 'MultiLineString':
+      return 0;
+    case 'GeometryCollection':
+      for (i = 0; i < _.geometries.length; i++) {
+        area += geometry(_.geometries[i]);
+      }
+      return area;
+    case 'Feature':
+      return geometry(_.geometry);
+    case 'FeatureCollection':
+      for (i = 0; i < _.features.length; i++) {
+        area += geometry(_.features[i]);
+      }
+      return area;
+    default:
+      return 0;
+  }
 }
 
 function polygonArea(coords) {
-    var area = 0;
-    if (coords && coords.length > 0) {
-        area += Math.abs(ringArea(coords[0]));
-        for (var i = 1; i < coords.length; i++) {
-            area -= Math.abs(ringArea(coords[i]));
-        }
+  let area = 0;
+  if (coords && coords.length > 0) {
+    area += Math.abs(ringArea(coords[0]));
+    for (let i = 1; i < coords.length; i++) {
+      area -= Math.abs(ringArea(coords[i]));
     }
-    return area;
+  }
+  return area;
 }
 
 /**
@@ -53,37 +64,46 @@ function polygonArea(coords) {
  */
 
 function ringArea(coords) {
-    var p1, p2, p3, lowerIndex, middleIndex, upperIndex, i,
-    area = 0,
-    coordsLength = coords.length;
+  let p1,
+    p2,
+    p3,
+    lowerIndex,
+    middleIndex,
+    upperIndex,
+    i,
+    area = 0;
+  const coordsLength = coords.length;
 
-    if (coordsLength > 2) {
-        for (i = 0; i < coordsLength; i++) {
-            if (i === coordsLength - 2) {// i = N-2
-                lowerIndex = coordsLength - 2;
-                middleIndex = coordsLength -1;
-                upperIndex = 0;
-            } else if (i === coordsLength - 1) {// i = N-1
-                lowerIndex = coordsLength - 1;
-                middleIndex = 0;
-                upperIndex = 1;
-            } else { // i = 0 to N-3
-                lowerIndex = i;
-                middleIndex = i+1;
-                upperIndex = i+2;
-            }
-            p1 = coords[lowerIndex];
-            p2 = coords[middleIndex];
-            p3 = coords[upperIndex];
-            area += ( rad(p3[0]) - rad(p1[0]) ) * Math.sin( rad(p2[1]));
-        }
-
-        area = area * wgs84.RADIUS * wgs84.RADIUS / 2;
+  if (coordsLength > 2) {
+    for (i = 0; i < coordsLength; i++) {
+      if (i === coordsLength - 2) {
+        // i = N-2
+        lowerIndex = coordsLength - 2;
+        middleIndex = coordsLength - 1;
+        upperIndex = 0;
+      } else if (i === coordsLength - 1) {
+        // i = N-1
+        lowerIndex = coordsLength - 1;
+        middleIndex = 0;
+        upperIndex = 1;
+      } else {
+        // i = 0 to N-3
+        lowerIndex = i;
+        middleIndex = i + 1;
+        upperIndex = i + 2;
+      }
+      p1 = coords[lowerIndex];
+      p2 = coords[middleIndex];
+      p3 = coords[upperIndex];
+      area += (rad(p3[0]) - rad(p1[0])) * Math.sin(rad(p2[1]));
     }
 
-    return area;
+    area = (area * wgs84.RADIUS * wgs84.RADIUS) / 2;
+  }
+
+  return area;
 }
 
 function rad(_) {
-    return _ * Math.PI / 180;
-}
\ No newline at end of file
+  return (_ * Math.PI) / 180;
+}