add area calculation for donet polygon

geojson-utils.js

@@ -221,27 +221,32 @@
 
   // adapted from http://paulbourke.net/geometry/polyarea/javascript.txt
   gju.area = function (polygon) {
-    var area = 0;
-    // TODO: polygon holes at coordinates[1]
-    var points = polygon.coordinates[0];
-    var j = points.length - 1;
-    var p1, p2;
+    var areaTotal = 0;
+    var count = polygon.coordinates.length;
+    for(var polyIndex = 0; polyIndex < count; polyIndex++){
+        var area = 0;
+        var points = polygon.coordinates[polyIndex];
+        var j = points.length - 1;
+        var p1, p2;
+
+        for (var i = 0; i < points.length; j = i++) {
+        var p1 = {
+          x: points[i][1],
+          y: points[i][0]
+        };
+        var p2 = {
+          x: points[j][1],
+          y: points[j][0]
+        };
+        area += p1.x * p2.y;
+        area -= p1.y * p2.x;
+        }
 
-    for (var i = 0; i < points.length; j = i++) {
-      var p1 = {
-        x: points[i][1],
-        y: points[i][0]
-      };
-      var p2 = {
-        x: points[j][1],
-        y: points[j][0]
-      };
-      area += p1.x * p2.y;
-      area -= p1.y * p2.x;
+        area /= 2;
+        areaTotal += Math.pow(-1, polyIndex)*area;
     }
-
-    area /= 2;
-    return area;
+
+    return areaTotal;
   },
 
   // adapted from http://paulbourke.net/geometry/polyarea/javascript.txt

test.js

@@ -61,4 +61,15 @@ var multipoly = {"type": "MultiPolygon",
 if (!gju.pointInMultiPolygon(point,multipoly)) throw new Error();
 if (gju.pointInMultiPolygon(singlepoint,multipoly)) throw new Error();
 
+//test gju.area
+if(gju.area(box) !== 100) throw new Error();
+var donetBox = {
+  "type": "Polygon",
+    "coordinates": [
+      [ [0, 0], [10, 0], [10, 10], [0, 10] ],
+      [ [3, 3], [6, 3], [6, 6], [3, 6]  ]
+    ]
+}
+if(gju.area(donetBox) !== (100-9)) throw new Error();
+
 console.log('all passed')