Move event callback arguments to own method

src/Dropdownizer.js

@@ -312,16 +312,7 @@ class Dropdownize {
     this._el.selectedIndex = this._lastSelectedIndex;
 
     if (this._changeCallback) {
-      let data = Object.assign({}, listItem.dataset);
-
-      delete data.selected;
-
-      this._changeCallback({
-        type: "change",
-        target: this._ui.div,
-        selectedTarget: listItem,
-        data
-      });
+      this._changeCallback(this._callbackArgs(listItem, "change"));
     }
 
     if (!this._changeFromOriginalElement) {
@@ -332,6 +323,25 @@ class Dropdownize {
     return this;
   }
 
+  _callbackArgs(listItem, type) {
+    let data = Object.assign({index: this._lastSelectedIndex}, listItem.dataset),
+        out;
+
+    delete data.selected;
+
+    out = {
+      target: this._ui.div,
+      selectedTarget: listItem,
+      data
+    };
+
+    if (type) {
+      out.type = type;
+    }
+
+    return out;
+  }
+
   /**
    * Listens for change events.
    * @param   {Function}    callback The callback function to execute when a list item changes.