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Possible parsing error with destructuring assignment #23142
Description
- 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]);
Activity
mscdex commentedon Sep 28, 2018
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 => ais the entirety of the function and[x, y] = ...is a second argument being passed tofooor if it's a continuation of thea => 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 commentedon Sep 28, 2018
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 commentedon Sep 28, 2018
It does appear that Firefox does not have this issue.
/cc @nodejs/v8 ?
hashseed commentedon Sep 28, 2018
@gsathya
ArnauldChevallier commentedon Sep 28, 2018
@mscdex The intended behavior is that
[x, y] = [2, 1]is parsed as a second argument forfoo. We can force a correct parsing by adding parentheses:foo(a => a, ([x, y] = [2, 1]))(orfoo(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 commentedon Sep 28, 2018
Filed https://bugs.chromium.org/p/v8/issues/detail?id=8241 to track this
Hakerh400 commentedon Oct 23, 2018
Seems to be fixed in v8/v8@cd21f71
Trott commentedon Nov 24, 2018
Should we float v8/v8@cd21f71? @nodejs/v8-update
deps: V8: backport cd21f71f9cb5
deps: V8: backport cd21f71f9cb5
richardlau commentedon Jan 19, 2021
Fixed in v10.22.0 by #33862.