Enter and exit return empty by default.

README.md

@@ -8,7 +8,7 @@ API changes from D3 3.x:
 
 * The selection.data method, when called with arguments, now modifies the current selection to be the update selection, rather than returning a new selection. See [#2402](https://github.com/mbostock/d3/issues/2402).
 
-* The selection.enter and selection.exit selections are now simply fields (defined by selection.data), not methods. See [#2402](https://github.com/mbostock/d3/issues/2402).
+* The selection.enter and selection.exit methods now return empty selections if the selection has not yet been bound to data. (Previously, these methods were only defined after calling selection.data, and attempting to access them before binding to data would throw an error.) See [#2402](https://github.com/mbostock/d3/issues/2402).
 
 * The selection.data method, when called *without* arguments, now returns an array of data for all elements in the selection, not just the first group.
 

lib/d3/selection.js

@@ -302,7 +302,9 @@ Selection.prototype = {
     }
 
     var enterGroups = new Array(m),
-        exitGroups = new Array(m);
+        exitGroups = new Array(m),
+        enter,
+        exit;
 
     function bind(group, groupData, j) {
       var i,
@@ -331,7 +333,7 @@ Selection.prototype = {
 
         for (i = -1; ++i < m;) {
           if (!(node = nodeByKeyValue.get(keyValue = key.call(groupData, nodeData = groupData[i], i)))) {
-            enterGroup[i] = new Placeholder(nodeData);
+            enterGroup[i] = new EnterNode(nodeData);
           } else if (node !== true) { // no duplicate data key
             updateGroup[i] = node;
             node.__data__ = nodeData;
@@ -352,11 +354,11 @@ Selection.prototype = {
             node.__data__ = nodeData;
             updateGroup[i] = node;
           } else {
-            enterGroup[i] = new Placeholder(nodeData);
+            enterGroup[i] = new EnterNode(nodeData);
           }
         }
         for (; i < m; ++i) {
-          enterGroup[i] = new Placeholder(groupData[i]);
+          enterGroup[i] = new EnterNode(groupData[i]);
         }
         for (; i < n; ++i) {
           exitGroup[i] = group[i];
@@ -380,11 +382,21 @@ Selection.prototype = {
       }
     }
 
-    this.enter = new EnterSelection(enterGroups);
-    this.exit = new Selection(exitGroups);
+    enter = new EnterSelection(enterGroups);
+    exit = new Selection(exitGroups);
+    this.enter = function() { return enter; };
+    this.exit = function() { return exit; };
     return this;
   },
 
+  enter: function() {
+    return this.data(dataOf).enter();
+  },
+
+  exit: function() {
+    return this.data(dataOf).exit();
+  },
+
   datum: function(value) {
     return arguments.length
         ? this.property("__data__", value)
@@ -563,7 +575,7 @@ EnterSelection.prototype.insert = function(name, before) {
   return Selection.prototype.insert.call(this, name, before);
 };
 
-function Placeholder(data) {
+function EnterNode(data) {
   this.__data__ = data;
 }
 
@@ -690,6 +702,12 @@ function filteredListenerOf(listener, args) {
   };
 }
 
+function dataOf() {
+  var i = -1, n = this.length, data = new Array(n), node;
+  while (++i < n) if (node = this[i]) data[i] = node.__data__;
+  return data;
+}
+
 function ascending(a, b) {
   return a < b ? -1 : a > b ? 1 : a >= b ? 0 : NaN;
 }

test/d3/selection-test.js

@@ -19,3 +19,23 @@ tape("can subselect a child element", function(test) {
   test.equal(s.node(), document.querySelector("h1"), "that element is the expected element");
   test.end();
 });
+
+tape("the enter selection is initially empty", function(test) {
+  var document = jsdom.jsdom("<h1>hello</h1>"),
+      s = selection.select(document.documentElement).selectAll("h1"),
+      e = s.enter();
+  test.ok(e instanceof selection, "returns an instanceof selection");
+  test.equal(e.size(), 0, "the enter selection is empty");
+  test.equal(e, s.enter(), "the enter selection is sticky");
+  test.end();
+});
+
+tape("the exit selection is initially empty", function(test) {
+  var document = jsdom.jsdom("<h1>hello</h1>"),
+      s = selection.select(document.documentElement).selectAll("h1"),
+      e = s.exit();
+  test.ok(e instanceof selection, "returns an instanceof selection");
+  test.equal(e.size(), 0, "the exit selection is empty");
+  test.equal(e, s.exit(), "the exit selection is sticky");
+  test.end();
+});