Adds feature requested in first bullet point of phosphorjs#354

packages/widgets/src/contextmenu.ts

@@ -167,6 +167,18 @@ namespace ContextMenu {
      * The default rank is `Infinity`.
      */
     rank?: number;
+
+    /**
+     * Pass a selected node's dataset to an item's command.
+     *
+     * If this flag is set, when a contextmenu item is matched
+     * to the target of a contextmenu event, the key-value pairs
+     * in `target.dataset` (if present) will be added as
+     * properties of `item.args`.
+     *
+     * Only meaningful for items of type `command`.
+     */
+    passDataset?: boolean;
   }
 }
 
@@ -190,6 +202,11 @@ namespace Private {
      */
     rank: number;
 
+    /**
+     * Pass a selected node's dataset to an item's command.
+     */
+    passDataset: boolean;
+
     /**
      * The tie-breaking id for the item.
      */
@@ -203,7 +220,8 @@ namespace Private {
   function createItem(options: ContextMenu.IItemOptions, id: number): IItem {
     let selector = validateSelector(options.selector);
     let rank = options.rank !== undefined ? options.rank : Infinity;
-    return { ...options, selector, rank, id };
+    let passDataset = options.passDataset !== undefined ? options.passDataset : false;
+    return { ...options, selector, rank, passDataset, id };
   }
 
   /**
@@ -266,6 +284,11 @@ namespace Private {
           continue;
         }
 
+        // Pass the target's dataset to the matched item's args, if requested.
+        if (item.passDataset && 'dataset' in target) {
+          item.args = {...item.args, ...(target as any)['dataset']};
+        }
+
         // Add the matched item to the result for this DOM level.
         matches.push(item);