Don’t copy arrays.

src/array.js

@@ -0,0 +1,3 @@
+export default function(x) {
+  return Array.isArray(x) ? x : Array.from(x);
+}

src/selectAll.js

@@ -1,7 +1,8 @@
+import array from "./array.js";
 import {Selection, root} from "./selection/index.js";
 
 export default function(selector) {
   return typeof selector === "string"
       ? new Selection([document.querySelectorAll(selector)], [document.documentElement])
-      : new Selection([selector == null ? [] : Array.from(selector)], root);
+      : new Selection([selector == null ? [] : array(selector)], root);
 }

src/selection/data.js

@@ -1,5 +1,6 @@
 import {Selection} from "./index.js";
 import {EnterNode} from "./enter.js";
+import array from "../array.js";
 import constant from "../constant.js";
 
 var keyPrefix = "$"; // Protect against keys like “__proto__”.
@@ -78,10 +79,6 @@ function datum(node) {
   return node.__data__;
 }
 
-function arrayify(data) {
-  return Array.isArray(data) ? data : Array.from(data);
-}
-
 export default function(value, key) {
   if (!value) return Array.from(this, datum);
 
@@ -95,7 +92,7 @@ export default function(value, key) {
     var parent = parents[j],
         group = groups[j],
         groupLength = group.length,
-        data = arrayify(value.call(parent, parent && parent.__data__, j, parents)),
+        data = array(value.call(parent, parent && parent.__data__, j, parents)),
         dataLength = data.length,
         enterGroup = enter[j] = new Array(dataLength),
         updateGroup = update[j] = new Array(dataLength),

test/selection/each-test.js

@@ -20,6 +20,6 @@ tape("selection.each(function) skips missing elements", function(test) {
       two = document.querySelector("#two"),
       selection = d3.selectAll([, one,, two]).datum(function(d, i) { return "node-" + i; });
   test.equal(selection.each(function(d, i, nodes) { result.push(this, d, i, nodes); }), selection);
-  test.deepEqual(result, [one, "node-1", 1, [undefined, one, undefined, two], two, "node-3", 3, [undefined, one, undefined, two]]);
+  test.deepEqual(result, [one, "node-1", 1, [, one,, two], two, "node-3", 3, [, one,, two]]);
   test.end();
 });