Fix for null node in selection.data.

If a selection is missing elements, then selection.data would still attempt to
evaluate the optional key function for the missing elements, resulting in an
uncaught TypeError. For example:

  d3.selectAll("p")
    .select(function(d, i) { return i & 1 ? this : null; })
      .data([1, 2, 3], Number);

Now, any missing elements are automatically assigned to the exit selection.

src/selection/data.js

@@ -36,12 +36,14 @@ d3_selectionPrototype.data = function(value, key) {
           keyValue;
 
       for (i = -1; ++i < n;) {
-        if (nodeByKeyValue.has(keyValue = key.call(node = group[i], node.__data__, i))) {
-          exitNodes[i] = node; // duplicate selection key
-        } else {
-          nodeByKeyValue.set(keyValue, node);
+        if (node = group[i]) {
+          if (nodeByKeyValue.has(keyValue = key.call(node, node.__data__, i))) {
+            exitNodes[i] = node; // duplicate selection key
+          } else {
+            nodeByKeyValue.set(keyValue, node);
+          }
+          keyValues[i] = keyValue;
         }
-        keyValues[i] = keyValue;
       }
 
       for (i = -1; ++i < m;) {
@@ -55,7 +57,7 @@ d3_selectionPrototype.data = function(value, key) {
       }
 
       for (i = -1; ++i < n;) {
-        if (nodeByKeyValue.get(keyValues[i]) !== true) {
+        if (i in keyValues && nodeByKeyValue.get(keyValues[i]) !== true) {
           exitNodes[i] = group[i];
         }
       }

test/selection/data-test.js

@@ -252,6 +252,40 @@ suite.addBatch({
         assert.domEqual(exit[1][0], span[1][0]);
         assert.domNull(exit[1][1]);
       },
+      "does not evaluate the key function on null nodes": function(span) {
+        var node = span[0][0];
+        span[0][0] = null;
+
+        var update = span.data([1, 2], Number);
+        assert.isFalse(update.empty());
+        assert.equal(update.length, 2);
+        assert.equal(update[0].length, 2);
+        assert.equal(update[1].length, 2);
+        assert.domEqual(update[0][0], span[0][1]);
+        assert.domNull(update[0][1]);
+        assert.domEqual(update[1][0], span[1][1]);
+        assert.domNull(update[1][1]);
+
+        var enter = update.enter();
+        assert.equal(enter.length, 2);
+        assert.equal(enter[0].length, 2);
+        assert.equal(enter[1].length, 2);
+        assert.domNull(enter[0][0]);
+        assert.deepEqual(enter[0][1], {__data__: 2});
+        assert.domNull(enter[1][0]);
+        assert.deepEqual(enter[1][1], {__data__: 2});
+
+        var exit = update.exit();
+        assert.equal(exit.length, 2);
+        assert.equal(exit[0].length, 2);
+        assert.equal(exit[1].length, 2);
+        assert.domNull(exit[0][0]);
+        assert.domNull(exit[0][1]);
+        assert.domEqual(exit[1][0], span[1][0]);
+        assert.domNull(exit[1][1]);
+
+        span[0][0] = node;
+      },
       "handles keys that are in the default object's prototype chain": function(span) {
         // This also applies to the non-standard "watch" and "unwatch" in Mozilla Firefox.
         var update = span.data(["hasOwnProperty", "isPrototypeOf", "toLocaleString", "toString", "valueOf"], String);