Sort null nodes at the end.

Previously, null nodes were passed to the comparator and were indistinguishable
from non-null nodes with no bound data. For consistency with other selection
operators that skip null nodes, it seems preferrable to put null nodes at the
end of the selection rather than passing them to the comparator. Fixes d3#881.

d3.js

@@ -1805,7 +1805,7 @@
   function d3_selection_sortComparator(comparator) {
     if (!arguments.length) comparator = d3.ascending;
     return function(a, b) {
-      return comparator(a && a.__data__, b && b.__data__);
+      return !a - !b || comparator(a.__data__, b.__data__);
     };
   }
   d3_selectionPrototype.on = function(type, listener, capture) {

src/core/selection-sort.js

@@ -7,6 +7,6 @@ d3_selectionPrototype.sort = function(comparator) {
 function d3_selection_sortComparator(comparator) {
   if (!arguments.length) comparator = d3.ascending;
   return function(a, b) {
-    return comparator(a && a.__data__, b && b.__data__);
+    return (!a - !b) || comparator(a.__data__, b.__data__);
   };
 }

test/core/selection-sort-test.js

@@ -36,13 +36,34 @@ suite.addBatch({
       assert.deepEqual(span[2].map(data), [1, 10, 12, 30]);
       assert.deepEqual(span[3].map(data), [1, 10, 22, 40]);
     },
-    "treats null nodes as null data": function(span) {
-      var some = d3.selectAll("div:first-child span"), nulls = false;
-      some[0][0] = null;
-      some.sort(function(a, b) { if ((a === null) || (b === null)) ++nulls; return a - b; });
-      assert.isTrue(nulls > 0);
+    "sorts null nodes at the end of the selection": function() {
+      var span = d3.selectAll("div").selectAll("span"), nulls = 0;
+      d3.select(span[0][0]).remove();
+      span[0][0] = null;
+      span.sort(function(a, b) { if ((a === null) || (b === null)) ++nulls; return a - b; });
+      assert.equal(nulls, 0);
+
+      assert.isNull(span[0][3]);
       assert.domNull(span[0][0].previousSibling);
-      assert.deepEqual(some[0].slice(1).map(data), [3, 10, 21]);
+      assert.domEqual(span[0][1], span[0][0].nextSibling);
+      assert.domEqual(span[0][0], span[0][1].previousSibling);
+      assert.domEqual(span[0][2], span[0][1].nextSibling);
+      assert.domEqual(span[0][1], span[0][2].previousSibling);
+      assert.domNull(span[0][2].nextSibling);
+      assert.deepEqual(span[0].slice(0, -1).map(data), [3, 10, 21]);
+
+      for (var i = 1; i < 4; ++i) {
+        var d = span[i].parentNode.__data__;
+        assert.domNull(span[i][0].previousSibling);
+        assert.domEqual(span[i][1], span[i][0].nextSibling);
+        assert.domEqual(span[i][0], span[i][1].previousSibling);
+        assert.domEqual(span[i][2], span[i][1].nextSibling);
+        assert.domEqual(span[i][1], span[i][2].previousSibling);
+        assert.domEqual(span[i][3], span[i][2].nextSibling);
+        assert.domEqual(span[i][2], span[i][3].previousSibling);
+        assert.domNull(span[i][3].nextSibling);
+        assert.deepEqual(span[i].map(data), [1, 2 + d, 10, 20 + d].sort(d3.ascending));
+      }
     },
     "returns the current selection": function(span) {
       span = d3.select("body"); // https://github.com/tmpvar/jsdom/issues/277