Rule to catch trailing commas in function parameters

This construct:

function (a, b, c, ) {

}

Doesn’t appear to be caught by general parsing, or any rule that I could find. It’s not legal at all, though, and should be caught by the parser.

[ljharb]

@ilyavolodin why not? Why is object rest/spread an exception but nothing else in stage 3?

Something being in stage 3 means, that barring some massive surprise performance issue discovered by browsers, that it almost certainly will ship. Browsers begin implementing in stage 3, since it’s a stage 4 requirement that 2 browsers support something first. What this policy means is that eslint will be unable to lint code that works in half of the browsers - that doesn’t seem ideal.

[ljharb]

@ilyavolodin ES2016 was ratified in June, so it’s complete. However, the “yearly spec” is irrelevant - only what’s on Github matters, which is all the things that are stage 4 (which luckily doesn’t yet include any syntax changes).

As for babel-eslint, the creator of this tool (who i won’t mention so as not to bother them) went out of their way to convince eslint-config-airbnb to remove that parser as soon as it was no longer needed by our settings. If eslint core doesn’t support stage 3 features, then we won’t be ever able to rely on that parser moving forward.

As a member of TC39, I’m trying to say that the right time to implement parser semantics is stage 3. If eslint chooses not to do this, that’s fine, but that means you will be lagging behind the entire JS community.