The code
```js
linesInfo && linesInfo[type]
```
performs a lot of dynamic lookups on the `Boolean.prototype`, as the
*ToBoolean* operation let's `true` pass for `linesInfo` (which might
itself be concerning that this can be a boolean). Instead of the
coercion, the code should properly check for valid objects via `typeof`
and strict equality with `null` comparison.
This is a non-breaking performance fix.
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
nodesOut[nodesOut.length - 1]
```
where `nodesOut` can be an empty array, are bad for performance in Node.
In this particular case it's easy to restructure the code a bit to not
require the array access at all, but just track the current `tail` as we
go.
This is a non-breaking performance fix.
* Fix path.popContext() to not try to load "-1" from contexts array.
The current implement of popContext does
```js
this.setContext(this.contexts[this.contexts.length - 1]);
```
even if `this.contexts` can be empty, which causes it to lookup the
property `"-1"`, which is not found on the array itself and obviously
also not in the `Object.prototype` and the `Array.prototype`. However
since `"-1"` is not a valid array index, but has a valid integer
representation, this is a very expensive lookup in V8 (and probably
other engines too, but that is probably less relevant, since Babel
most often runs on Node nowadays).
* Make zero check explicit (for readability).
Similar in spirit to https://github.com/babel/babel/pull/6580, the
current implementation did
```js
node.params[node.params.length - 1]
```
where `node.params` can also be empty, which causes it to lookup the
property `"-1"`, which is not found on the array itself and obviously
also not in the `Object.prototype` and the `Array.prototype`. However
since `"-1"` is not a valid array index, but has a valid integer
representation, this is a very expensive lookup in V8 (and probably
other engines too, but that is probably less relevant, since Babel
most often runs on Node nowadays). In V8 this causes a call to
the `%SetProperty` runtime function for each of these `"-1"`
property lookups, and in addition sends the whole `KeyedLoadIC`
to `MEGAMORPHIC` state, which also penalizes other accesses
on this line.
This is a small non-breaking performance fix.
* Initial version
* Replace .includes with .indexOf and better node set to visit
* Alphabetically sort es3 reserved words and make difference helper for readability
* Fix second Array.include error that was not polyfilled
* Move es3 keywords into separate babel-types helper and use in all es3 transforms
* Reference local plugin build directly for tests
* Try relative import for babel-types
* Update to scoped package name and beta 3
* Fix unscoped package import
* Replace local plugin reference with proper plugin name
