df0d9d05a3
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.
The compiler for writing next generation JavaScript.
Supporting Babel
Babel is community-driven and thus mostly maintained by a group of volunteers. It has a lot of companies and projects using it but almost no sponsors/people funded to work on it. If you'd like to help maintain the future of the project, please consider:
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Intro
Babel is a tool that helps you write code in the latest version of JavaScript. When your supported environments don't support certain features natively, Babel will help you compile those features down to a supported version.
In
// ES2015 arrow function
[1, 2, 3].map((n) => n + 1);
Out
[1, 2, 3].map(function(n) {
return n + 1;
});
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FAQ
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For questions and support please visit join our Slack Community, ask a question on Stack Overflow, or ping us on Twitter.
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Check out our website: babeljs.io, and report issues/features at babel/website.
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Please read through our CONTRIBUTING.md and fill out the issue template at babel/issues!
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Check out our CONTRIBUTING.md to get started with setting up the repo.
- If you have already joined Slack, join our #development channel and say hi!
- Check out the issues with the good first issue and help wanted label. We suggest also looking at the closed ones to get a sense of the kinds of issues you can tackle.
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The Babel repo is managed as a monorepo that is composed of many npm packages.
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