This is not actually about Tail Call Optimisation, which is more flexible and optional matter of optimisation, but about Proper Tail Calls, which are a guarante

This is not actually about Tail Call Optimisation, which is more flexible and optional matter of optimisation, but about Proper Tail Calls, which are a guarantee made in the ECMAScript 6 specification (over implementer concerns objections)—in strict mode, calls in tail position must not create additional stack frames. This is the last piece of ECMAScript 6 that most engines haven’t implemented, because it’s rather controversial: it actually causes some performance problems, and makes debugging harder, and may have security issues (in 2016, Mozilla declared it impossible to implement across realm boundaries due to their security model). https://github.com/tc39/proposal-ptc-syntax https://github.com/tc39/proposal-ptc-syntax has a lot of useful information about it all, and a proposal to make it explicit in syntax, such as with `return continue …`, though I don’t believe that’s really gone anywhere. The practical situation is that this is the one part of the ECMAScript spec that most implementers ignore, and thus which you can’t depend on. Which says to me that it should either be made optional or be removed from the spec. Not sure if there are any other features similarly disregarded. ECMAScript specification policy was different in those days, they operated more independently of implementers. (HTML was like that once too—that’s roughly why WHATWG was formed, because the actual implementers weren’t happy with how things worked in W3C, so they took matters into their own hands.) (Fun terminology problems here. The term TCO is commonly used for PTC, and PTC is very close to being a subset of TCO, but the mandatory stack frame elision which ruins debugging feels to me like it falls outside of TCO. In various situations, debuggers will mark things like “stack frame omitted” when they’ve optimised one out of existence, but you can generally compile things differently, or something like that, to prevent this. (Compiled/dynamic language variation showing up here.) But with PTC, it feels like the engine is kinda not even allowed to know that a stack frames may be absent. So I say PTC and TCO are a little distinct, though PTC is mostly just a subset of TCO. Reminds me of the terminology of tree-shaking versus dead code removal—where the former is essentially a subset of the latter, but that the effects are just slightly different, though I’d say it’s more slight in that case than this.)