That's a declaration. That would be cool. Doesn't currently work with let or var either.

I think this is basically assigning values to the window object, similar to updating values in any object.

e.g.

    const a = { b: 1 }
    console.log(a.b = 2); // logs 2

Only if you haven't already declared the variable. e.g.

    let x;
    console.log(x = 'hello');

My point is that the `x = 'hello'` part is an assignment, and an expression, and has the exact same semantics regardless of whether you use that expression as a statement. So, assignments would seem to be expressions.