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Fix "-1" array accesses in CommentsParser. (#777)

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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.
This commit is contained in:
Benedikt Meurer 2017-10-28 22:07:28 +02:00 committed by Henry Zhu
parent e4bcd1d0ff
commit 55d5545614
1 changed files with 10 additions and 8 deletions
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18
src/parser/comments.js
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@ -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;