Mutability is distinct from variability. In Javascript only because it's a pretty widely known syntax:

    const f = (x) => {
      const y = x + 1;
      return y + 1;
    }

I've only glanced at this Zen-C thing but I presume it's the same story.

"immutable variable" is an oxymoron. Just because Javascript did it does not mean every new language has to do it the same way.