Pass index when traversing.

README.md

@@ -174,17 +174,17 @@ root
 
 You must call *node*.sort before invoking a hierarchical layout if you want the new sort order to affect the layout; see [*node*.sum](#node_sum) for an example.
 
-<a name="node_each" href="#node_each">#</a> <i>node</i>.<b>each</b>(<i>function</i>) · [Source](https://github.com/d3/d3-hierarchy/blob/master/src/hierarchy/each.js), [Examples](https://observablehq.com/@d3/visiting-a-d3-hierarchy)
+<a name="node_each" href="#node_each">#</a> <i>node</i>.<b>each</b>(<i>function</i>[, <i>that</i>]) · [Source](https://github.com/d3/d3-hierarchy/blob/master/src/hierarchy/each.js), [Examples](https://observablehq.com/@d3/visiting-a-d3-hierarchy)
 
-Invokes the specified *function* for *node* and each descendant in [breadth-first order](https://en.wikipedia.org/wiki/Breadth-first_search), such that a given *node* is only visited if all nodes of lesser depth have already been visited, as well as all preceding nodes of the same depth. The specified function is passed the current *node*.
+Invokes the specified *function* for *node* and each descendant in [breadth-first order](https://en.wikipedia.org/wiki/Breadth-first_search), such that a given *node* is only visited if all nodes of lesser depth have already been visited, as well as all preceding nodes of the same depth. The specified function is passed the current *descendant*, the zero-based traversal *index*, and this *node*. If *that* is specified, it is the this context of the callback.
 
-<a name="node_eachAfter" href="#node_eachAfter">#</a> <i>node</i>.<b>eachAfter</b>(<i>function</i>) · [Source](https://github.com/d3/d3-hierarchy/blob/master/src/hierarchy/eachAfter.js), [Examples](https://observablehq.com/@d3/visiting-a-d3-hierarchy)
+<a name="node_eachAfter" href="#node_eachAfter">#</a> <i>node</i>.<b>eachAfter</b>(<i>function</i>[, <i>that</i>]) · [Source](https://github.com/d3/d3-hierarchy/blob/master/src/hierarchy/eachAfter.js), [Examples](https://observablehq.com/@d3/visiting-a-d3-hierarchy)
 
-Invokes the specified *function* for *node* and each descendant in [post-order traversal](https://en.wikipedia.org/wiki/Tree_traversal#Post-order), such that a given *node* is only visited after all of its descendants have already been visited. The specified function is passed the current *node*.
+Invokes the specified *function* for *node* and each descendant in [post-order traversal](https://en.wikipedia.org/wiki/Tree_traversal#Post-order), such that a given *node* is only visited after all of its descendants have already been visited. The specified function is passed the current *descendant*, the zero-based traversal *index*, and this *node*. If *that* is specified, it is the this context of the callback.
 
-<a name="node_eachBefore" href="#node_eachBefore">#</a> <i>node</i>.<b>eachBefore</b>(<i>function</i>) · [Source](https://github.com/d3/d3-hierarchy/blob/master/src/hierarchy/eachBefore.js), [Examples](https://observablehq.com/@d3/visiting-a-d3-hierarchy)
+<a name="node_eachBefore" href="#node_eachBefore">#</a> <i>node</i>.<b>eachBefore</b>(<i>function</i>[, <i>that</i>]) · [Source](https://github.com/d3/d3-hierarchy/blob/master/src/hierarchy/eachBefore.js), [Examples](https://observablehq.com/@d3/visiting-a-d3-hierarchy)
 
-Invokes the specified *function* for *node* and each descendant in [pre-order traversal](https://en.wikipedia.org/wiki/Tree_traversal#Pre-order), such that a given *node* is only visited after all of its ancestors have already been visited. The specified function is passed the current *node*.
+Invokes the specified *function* for *node* and each descendant in [pre-order traversal](https://en.wikipedia.org/wiki/Tree_traversal#Pre-order), such that a given *node* is only visited after all of its ancestors have already been visited. The specified function is passed the current *descendant*, the zero-based traversal *index*, and this *node*. If *that* is specified, it is the this context of the callback.
 
 <a name="node_copy" href="#node_copy">#</a> <i>node</i>.<b>copy</b>() · [Source](https://github.com/d3/d3-hierarchy/blob/master/src/hierarchy/index.js), [Examples](https://observablehq.com/@d3/d3-hierarchy)
 

src/hierarchy/each.js

@@ -1,11 +1,13 @@
-export default function(callback) {
-  var node = this, current, next = [node], children, i, n;
+export default function(callback, that) {
+  var node = this, current, next = [node], children, i, n, index = -1;
   do {
     current = next.reverse(), next = [];
     while (node = current.pop()) {
-      callback(node), children = node.children;
-      if (children) for (i = 0, n = children.length; i < n; ++i) {
-        next.push(children[i]);
+      callback.call(that, node, ++index, this);
+      if (children = node.children) {
+        for (i = 0, n = children.length; i < n; ++i) {
+          next.push(children[i]);
+        }
       }
     }
   } while (next.length);

src/hierarchy/eachAfter.js

@@ -1,13 +1,15 @@
-export default function(callback) {
-  var node = this, nodes = [node], next = [], children, i, n;
+export default function(callback, that) {
+  var node = this, nodes = [node], next = [], children, i, n, index = -1;
   while (node = nodes.pop()) {
-    next.push(node), children = node.children;
-    if (children) for (i = 0, n = children.length; i < n; ++i) {
-      nodes.push(children[i]);
+    next.push(node);
+    if (children = node.children) {
+      for (i = 0, n = children.length; i < n; ++i) {
+        nodes.push(children[i]);
+      }
     }
   }
   while (node = next.pop()) {
-    callback(node);
+    callback.call(that, node, ++index, this);
   }
   return this;
 }

src/hierarchy/eachBefore.js

@@ -1,9 +1,11 @@
-export default function(callback) {
-  var node = this, nodes = [node], children, i;
+export default function(callback, that) {
+  var node = this, nodes = [node], children, i, index = -1;
   while (node = nodes.pop()) {
-    callback(node), children = node.children;
-    if (children) for (i = children.length - 1; i >= 0; --i) {
-      nodes.push(children[i]);
+    callback.call(that, node, ++index, this);
+    if (children = node.children) {
+      for (i = children.length - 1; i >= 0; --i) {
+        nodes.push(children[i]);
+      }
     }
   }
   return this;