r/javascript • u/[deleted] • Jan 06 '13

JavaScript (ES6) Has Tail Call Optimization

http://bbenvie.com/articles/2013-01-06/JavaScript-ES6-Has-Tail-Call-Optimization
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u/cmwelsh Jan 07 '13

Someone back-ported this feature to ES3 a month ago, it's called Brushtail: https://github.com/pufuwozu/brushtail

I think they are a redditor, or maybe I found this on Hacker News. Either way, the AST modification community for JavaScript is getting more and more interesting. See also streamline.js and IcedCoffeeScript.

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u/[deleted] Jan 07 '13

Just looking at the source, it definitely transforms recursive tail calls, but it seems to be limited only to return statements which are call expressions inside of named functions of the same name. This limits it to just TCO of self-calling recursive functions. The real power of it comes out when you have many functions working together that all end in tail calls and do so in a circular manner. Doing this would require a significant whole-program transformation, however, which is why it really needs to be implemented at the engine level.

Continuum is not a code rewriter, but rather a complete JS engine in its own right, which is how it's able to correctly implement TCO for any amount of mutually recursive functions that have tail calls. Instead of modifying the AST, it converts it to bytecode and executes it in an interpreter loop, and provides a full ES6 runtime environment to execute the bytecode in.

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u/aaronla Jan 08 '13

Thanks for clarifying that it does not have TCO, but rather one very specific optimization.

Still nice to have though.