Feature: Don't care/anonymous parameter

[qc00]

Many functional languages have a don't care parameter/variable that holds values not used by programs.

For example, a function like Array.prototype.forEach supplies three arguments to the callback. If the code only needs the second argument (index), it will still have to supply a name for the first one. The programmer can use a don't care parameter as the name of the first argument instead. This improves readability and avoids problem of accidentally shadowing.

An additional keyword/comment may have to be introduced to define which string is used for this purpose as the traditional "_" happens to be used by the popular underscore library.

This can also be extended to destructuring code.

[RyanCavanaugh]

Just thinking aloud -- would arr.forEach(( , index) => console.log(index)); look too much like a typo?

[sandersn]

I think so. Also, when commas stack up they get hard to count.

Does the already-supported workaround of _, __, __, ... suffice?

[FabianLauer]

I agree with @sandersn. How about arr.forEach((void, index) => console.log(index));?
Just the first thing that came to mind. Correct me if I'm wrong, but I think this doesn't lead to a conflict with the ES spec as it would be a part of the type system.

[coreh]

Does the already-supported workaround of _, __, __, ... suffice?

Maybe _ could be special cased, so that you can use it multiple times, then __ and ___ wouldn't be needed.

(_, __, x, ____) => x + 1

Would become

(_, _, x, _) => x + 1

[LiamGoodacre]

How about ?? For example: (?, x, ?) => x + 1.
Given that _ is a valid identifier name, I don't believe it is appropriate to use here.

[DanielRosenwasser]

You could just use an empty binding pattern like[] or {}.

arr.forEach(({}, index) => console.log(index));

arr.forEach(([], index) => console.log(index));

Since they are not disallowed (see tc39/ecma262#97), I'm inclined to close this unless there's a problem with that workaround.

[LiamGoodacre]

@DanielRosenwasser nice 👍

[qc00]

@DanielRosenwasser Sounds perfect. Would be nice if this special use case is documented though.

[FabianLauer]

@DanielRosenwasser Great, thanks.