Fix: destructuring assignment with an empty array in object

[zdenko]

While testing sub-structuring (#4995) I noticed wrong output of destructuring with an empty array in the object.

Current branch:

{a:[], b} = obj

###
var b;
obj.undefined, b = obj[1];
###

Since @variable.isAssignable() in Assign returns false because of an empty array, the ::compileDestructuring is invoked.
The syntax is valid and can be output directly.

This PR:

{a:[], b} = obj

###
var b;
({
  a: [],
  b
} = c);
###

[aminland]

This would be a breaking change since babel runs {a: []} through function _toArray(arr) { return Array.isArray(arr) ? arr : Array.from(arr); }.

As an example Array.from(null) and Array.from(undefined) throws a type error, so anyone who was using it to drop keys will be screwed.

[zdenko]

I think my comment from #4995 applies here as well.
I see no difference between {a: []} = b or {a:{}} = b. It is upon user to take care of destructuring variables.
And, I don't think anyone will be screwed, because currently, if an empty array is used in the destructuring`, you won't get correct result even if the key exists in the object.

[GeoffreyBooth]

I don’t think we should care what Babel does. If we output valid JS, that’s all we need to worry about.

This is correct, isn’t it?

obj = {a: [1]}
result = ({a: []} = obj)

In Chrome console, result equals {a: [1]}.

So @zdenko and @aminland is this safe to merge in?

[zdenko]

@GeoffreyBooth go for it.