Visitor: fix description of "ancestors" arg + test (#1250)

* Visitor: fix description of "ancestors" arg + test

* Make test more explicit

src/language/__tests__/visitor-test.js

@@ -74,7 +74,6 @@ describe('Visitor', () => {
         checkVisitorFnArgs(ast, arguments);
         visited.push(['enter', path.slice()]);
       },
-
       leave(node, key, parent, path) {
         checkVisitorFnArgs(ast, arguments);
         visited.push(['leave', path.slice()]);
@@ -95,6 +94,34 @@ describe('Visitor', () => {
     ]);
   });
 
+  it('validates ancestors argument', () => {
+    const ast = parse('{ a }', { noLocation: true });
+    const visitedNodes = [];
+
+    visit(ast, {
+      enter(node, key, parent, path, ancestors) {
+        const inArray = typeof key === 'number';
+        if (inArray) {
+          visitedNodes.push(parent);
+        }
+        visitedNodes.push(node);
+
+        const expectedAncestors = visitedNodes.slice(0, -2);
+        expect(ancestors).to.deep.equal(expectedAncestors);
+      },
+      leave(node, key, parent, path, ancestors) {
+        const expectedAncestors = visitedNodes.slice(0, -2);
+        expect(ancestors).to.deep.equal(expectedAncestors);
+
+        const inArray = typeof key === 'number';
+        if (inArray) {
+          visitedNodes.pop();
+        }
+        visitedNodes.pop();
+      },
+    });
+  });
+
   it('allows editing a node both on enter and on leave', () => {
     const ast = parse('{ a, b, c { a, b, c } }', { noLocation: true });
 

src/language/visitor.js

@@ -40,9 +40,9 @@ export type VisitFn<TAnyNode, TVisitedNode: TAnyNode = TAnyNode> = (
   parent: TAnyNode | $ReadOnlyArray<TAnyNode> | void,
   // The key path to get to this node from the root node.
   path: $ReadOnlyArray<string | number>,
-  // All nodes and Arrays visited before reaching this node.
+  // All nodes and Arrays visited before reaching parent of this node.
   // These correspond to array indices in `path`.
-  // Note: ancestors includes arrays which contain the visited node.
+  // Note: ancestors includes arrays which contain the parent of visited node.
   ancestors: $ReadOnlyArray<TAnyNode | $ReadOnlyArray<TAnyNode>>,
 ) => any;