more defensive copy selectAll

src/array.js

@@ -1,5 +1,3 @@
-export default function(x) {
-  return typeof x === "object" && "length" in x
-    ? x // Array, TypedArray, NodeList, array-like
-    : Array.from(x); // Map, Set, iterable, string, or anything else
+export default function array(x) {
+  return x == null ? [] : Array.isArray(x) ? x : Array.from(x);
 }

src/selectAll.js

@@ -4,5 +4,5 @@ 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(selector)], root);
+      : new Selection([array(selector)], root);
 }

src/selection/data.js

@@ -1,6 +1,5 @@
 import {Selection} from "./index.js";
 import {EnterNode} from "./enter.js";
-import array from "../array.js";
 import constant from "../constant.js";
 
 function bindIndex(parent, group, enter, update, exit, data) {
@@ -90,7 +89,7 @@ export default function(value, key) {
     var parent = parents[j],
         group = groups[j],
         groupLength = group.length,
-        data = array(value.call(parent, parent && parent.__data__, j, parents)),
+        data = arraylike(value.call(parent, parent && parent.__data__, j, parents)),
         dataLength = data.length,
         enterGroup = enter[j] = new Array(dataLength),
         updateGroup = update[j] = new Array(dataLength),
@@ -115,3 +114,15 @@ export default function(value, key) {
   update._exit = exit;
   return update;
 }
+
+// Given some data, this returns an array-like view of it: an object that
+// exposes a length property and allows numeric indexing. Note that unlike
+// selectAll, this isn’t worried about “live” collections because the resulting
+// array will only be used briefly while data is being bound. (It is possible to
+// cause the data to change while iterating by using a key function, but please
+// don’t; we’d rather avoid a gratuitous copy.)
+function arraylike(data) {
+  return typeof data === "object" && "length" in data
+    ? data // Array, TypedArray, NodeList, array-like
+    : Array.from(data); // Map, Set, iterable, string, or anything else
+}

src/selection/selectAll.js

@@ -4,8 +4,7 @@ import selectorAll from "../selectorAll.js";
 
 function arrayAll(select) {
   return function() {
-    var group = select.apply(this, arguments);
-    return group == null ? [] : array(group);
+    return array(select.apply(this, arguments));
   };
 }