comma operator with declarations in a for-loop

Question

In the following JavaScript code block I do NOT want to declare a new function f in the for-scope but just to assign () => i to the previously declared let f, thus creating a closure to the for-scoped declared and defined variable i.

Unfortunately the code results in a Type Error: f is not a function because f = () => i is being interpreted as let f = () => i:

{
    let f;
    for (let i = 'a', f = () => i; i == 'a'; ) {
        i = 'b';
    }
    f();
}

In the for-loop, how can I separate f = () => i from the precedent let i = 'a'?

Putting it in parentheses results in a Syntax Error:

{
    let f;
    for ((let i = 'a'), f = () => i; i == 'a'; ) {
        i = 'b';
    }
    f();
}

I don't want to change the scopes. I'm just looking for a syntactic mean to express the exactly scope constellation as given in my question.

Answer 1

I've found the solution:

{
    let f;
    for (let i = (f = () => i, 'a'); i == 'a'; ) {
        i = 'b';
    }
    f();
}

Or with an additional dummy-helper:

{
    let f;
    for (let i = 'a', dummy = (f = () => i, 'ignore'); i == 'a'; ) {
        i = 'b';
    }
    f();
}