Possible parsing error with destructuring assignment

[ArnauldChevallier]

• Version: 10.9.0
• Platform 1: Fedora 28 (Cloud Edition) x86_64
• Platform 2: Windows 10 x86_64
• Subsystem: n/a

Summary

Passing an arrow function and a destructuring assignment as two consecutive parameters of a function causes a syntax error.

Description

The following piece of code does not make much sense (and putting the initialization of [x, y] in the 2nd parameter of map() is definitely not a good idea), but I'd expect it to work anyway:

[[3, 4]].map(([X, Y]) => [X + x, Y + y], [x, y] = [2, 1])

In fact, this does work on both SpiderMonkey and Chakra which are returning [[5, 5]].

In Node (and Chrome), however, we get the following error:

SyntaxError: Invalid destructuring assignment target

Worst yet, with the following code:

[].map(_ => _, [x, y] = [2, 1])

We now get the following error:

SyntaxError: Unexpected token =>

This message is inconsistent, and that's why I suspect some kind of parsing error here.

Some more examples

This works:

foo = a => a;
foo(0, [x, y] = [2, 1]);

This also works:

foo = a => a;
foo(a => a, x = [2, 1]);

But this fails with "SyntaxError: Unexpected token =>":

foo = a => a;
foo(a => a, [x, y] = [2, 1]);

[mscdex]

This is a language-level issue and not node-specific.

The problem is that without braces, there is some ambiguity, since if you add them it is parsed just fine. The parser doesn't know whether the a => a is the entirety of the function and [x, y] = ... is a second argument being passed to foo or if it's a continuation of the a => a, ... function argument. By adding braces you are breaking that ambiguity: foo(a => {a, [x, y] = [2, 1]});.

FWIW this is why I personally always use braces (and semicolons to avoid that class of issues) to avoid such tricky behavior with the language.

[Hakerh400]

Minimal repro: a(a=>a,[]=a)
This seems like a spec-violation to me.
Since the same behavior is seen in Chrome, if it is a bug, it is a v8 bug and has nothing to do with Node.js

@ArnauldChevallier
Can you please report it to https://bugs.chromium.org/p/v8/issues/list

[mscdex]

It does appear that Firefox does not have this issue.

/cc @nodejs/v8 ?

[hashseed]

@gsathya

[ArnauldChevallier]

@mscdex The intended behavior is that [x, y] = [2, 1] is parsed as a second argument for foo. We can force a correct parsing by adding parentheses: foo(a => a, ([x, y] = [2, 1])) (or foo(a => { return a }, ([x, y] = [2, 1])) with braces). Doing assignments within passed parameters can be considered a dubious practice, but I don't think there's anything wrong with this syntax with strict regards to the spec.

Now, this does indeed look like a V8 issue rather than a Node.js one.

[gsathya]

Filed https://bugs.chromium.org/p/v8/issues/detail?id=8241 to track this

[Hakerh400]

Seems to be fixed in v8/v8@cd21f71

[Trott]

Should we float v8/v8@cd21f71? @nodejs/v8-update

[richardlau]

Fixed in v10.22.0 by #33862.