> for the longest time for loops required a mutating index

Well, at least one mutating index is convenient:

  function forEach (a,f) {  
    for(var i=0;i<a.length;i++) f(a[i],i,a);
  }

After that, you generally wouldn't need more indexes.

But does it really require a mutating index?

  function forEach (a,f,i) {
    var n = i || 0;
    f(a[0],n,a);
    if(a.length > 1)
      forEach(a.slice(1),f,n+1);
  }

Array copies aren't particularly efficient, of course, and there are languages with data structures backing arrays for which some similar approach might be efficient, but there it is.