update README; add test; remove obsolete instanceof

Author: mbostock

README.md

@@ -198,7 +198,7 @@ The returned filtered selection preserves the parents of this selection, but lik
 
 <a name="selection_merge" href="#selection_merge">#</a> <i>selection</i>.<b>merge</b>(<i>other</i>) · [Source](https://github.com/d3/d3-selection/blob/master/src/selection/merge.js)
 
-Returns a new selection merging this selection with the specified *other* selection. The returned selection has the same number of groups and the same parents as this selection. Any missing (null) elements in this selection are filled with the corresponding element, if present (not null), from the specified *selection*. (If the *other* selection has additional groups or parents, they are ignored.)
+Returns a new selection merging this selection with the specified *other* selection or transition. The returned selection has the same number of groups and the same parents as this selection. Any missing (null) elements in this selection are filled with the corresponding element, if present (not null), from the specified *selection*. (If the *other* selection has additional groups or parents, they are ignored.)
 
 This method is used internally by [*selection*.join](#selection_join) to merge the [enter](#selection_enter) and [update](#selection_data) selections after [binding data](#joining-data). You can also merge explicitly, although note that since merging is based on element index, you should use operations that preserve index, such as [*selection*.select](#selection_select) instead of [*selection*.filter](#selection_filter). For example:
 

src/selection/merge.js

@@ -2,7 +2,6 @@ import {Selection} from "./index.js";
 
 export default function(context) {
   var selection = context.selection ? context.selection() : context;
-  if (!(selection instanceof Selection)) throw new Error("invalid merge");
 
   for (var groups0 = this._groups, groups1 = selection._groups, m0 = groups0.length, m1 = groups1.length, m = Math.min(m0, m1), merges = new Array(m0), j = 0; j < m; ++j) {
     for (var group0 = groups0[j], group1 = groups1[j], n = group0.length, merge = merges[j] = new Array(n), node, i = 0; i < n; ++i) {

test/selection/merge-test.js

@@ -44,3 +44,20 @@ it("selection.merge(selection) reuses this selection’s parents", "<parent><chi
   assert.strictEqual(selection01._parents, selection0._parents);
   assert.strictEqual(selection10._parents, selection1._parents);
 });
+
+it("selection.merge(transition) returns a new selection, merging the two selections", "<h1 id='one'>one</h1><h1 id='two'>two</h1>", () => {
+  const one = document.querySelector("#one");
+  const two = document.querySelector("#two");
+  const selection0 = select(document.body).selectAll("h1");
+  const selection1 = selection0.select(function(d, i) { return i & 1 ? this : null; });
+  const selection2 = selection0.select(function(d, i) { return i & 1 ? null : this; });
+  assertSelection(selection1.merge(mockTransition(selection2)), {groups: [[one, two]], parents: [document.body]});
+});
+
+function mockTransition(selection) {
+  return {
+    selection() {
+      return selection;
+    }
+  };
+}