why not just

    function baz(modifier) {
       let foo = do_a();
       let bar = modifier(foo);
       return do_b(bar);
    }

    Example 1: baz((foo) => foo);
    Example 2: baz((foo) => do_z(foo));
    Example 3: baz((foo) => foo + 10);

which lets you be more concise with

    baz = (modifier) => do_b(modifier(do_a()))