Also

for (var a = (x = b in c) => {};;);

I love this content, keep it coming :P ❤️

Sounds like a duplicate of #9978. Or is there a reason to write code like that?

This is about a for loop, not a for-in loop.

What would be the correct js output here? Writing this as pure javascript returns the same error

The correct JS output here is the same as the input. This code throws ReferenceError at runtime but it isn't a parse error.

@jesus-diaz-rivero If you want some code that does not throw at runtime:

var b = "";
var c = {};
for (var a = [b in c]; false;);