From 55d5545614782ee87de47cd8be6898ef91d95c97 Mon Sep 17 00:00:00 2001
From: Benedikt Meurer <benedikt.meurer@googlemail.com>
Date: Sat, 28 Oct 2017 22:07:28 +0200
Subject: [PATCH] Fix "-1" array accesses in CommentsParser. (#777)

Similar to the fixes in https://github.com/babel/babel/pull/6580 and
https://github.com/babel/babel/pull/6581, accesses of the form

```js
stack[stack.length - 1];
```

when `stack` can be an empty array are pretty bad for performance.
In this case it also breaks the type safety, since the function
`last<T>` is declared to only return values of type `T`, but
occasionally also returns `undefined` now, since the `stack` parameters
passed to it never contain a property `"-1"` and neither do the
`Object.prototype` or the `Array.prototype`.

This is a non-breaking performance fix, which adds proper checking
to ensure that `last` is only invoked on non-empty arrays.
---
 src/parser/comments.js | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/src/parser/comments.js b/src/parser/comments.js
index b3d4925927..6bc7e2b2f9 100644
--- a/src/parser/comments.js
+++ b/src/parser/comments.js
@@ -65,14 +65,15 @@ export default class CommentsParser extends BaseParser {
         this.state.trailingComments.length = 0;
       }
     } else {
-      const lastInStack = last(stack);
-      if (
-        stack.length > 0 &&
-        lastInStack.trailingComments &&
-        lastInStack.trailingComments[0].start >= node.end
-      ) {
-        trailingComments = lastInStack.trailingComments;
-        lastInStack.trailingComments = null;
+      if (stack.length > 0) {
+        const lastInStack = last(stack);
+        if (
+          lastInStack.trailingComments &&
+          lastInStack.trailingComments[0].start >= node.end
+        ) {
+          trailingComments = lastInStack.trailingComments;
+          lastInStack.trailingComments = null;
+        }
       }
     }
 
@@ -138,6 +139,7 @@ export default class CommentsParser extends BaseParser {
       if (lastChild.leadingComments) {
         if (
           lastChild !== node &&
+          lastChild.leadingComments.length > 0 &&
           last(lastChild.leadingComments).end <= node.start
         ) {
           node.leadingComments = lastChild.leadingComments;