allow setting of keys (ordering) in before modifier

index.js

@@ -253,6 +253,7 @@ function walk (root, cb, immutable) {
         // use return values to update if defined
         var ret = cb.call(state, state.node);
         if (ret !== undefined && state.update) state.update(ret);
+        state.keys = null;
         if (modifiers.before) modifiers.before.call(state, state.node);
 
         if (!keepGoing) return state;
@@ -261,7 +262,7 @@ function walk (root, cb, immutable) {
         && state.node !== null && !state.circular) {
             parents.push(state);
 
-            var keys = Object.keys(state.node);
+            var keys = state.keys || Object.keys(state.node);
             keys.forEach(function (key, i) {
                 path.push(key);
 

test/keys.js

@@ -0,0 +1,30 @@
+var assert = require('assert');
+var Traverse = require('traverse');
+
+exports['sort test'] = function () {
+    var acc = [];
+    Traverse({
+        a: 30,
+        b: 22,
+        id: 9
+    }).forEach(function (node) {
+        if ((! Array.isArray(node)) && typeof node === 'object') {
+            this.before(function(node) {
+                this.keys = Object.keys(node);
+                this.keys.sort(function(a, b) {
+                    a = [a === "id" ? 0 : 1, a];
+                    b = [b === "id" ? 0 : 1, b];
+                    return a < b ? -1 : a > b ? 1 : 0;
+                });
+            });
+        }
+        if (this.isLeaf) acc.push(node);
+    });
+
+    assert.equal(
+        acc.join(' '),
+        '9 30 22',
+        'Traversal in a custom order'
+    );
+};
+