• 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]);