Question

<!-- first -->
<script>
var total = 0;
var newAccumulator = function()
{
  return function(i) { total += i; };
}

var foldl = function(arr, putNum)
{
  for (var i = 0; i < arr.length; ++i)
  {
    putNum(arr[i]);
  }
}

foldl([1, 2, 3, 4], newAccumulator());
document.write("Sum: " + total + "<br/>");
</script>

<!-- second -->
<script>
var total = 0;
var newAccumulator = function(i)
{
  total += i;
}

var foldl = function(arr, putNum)
{
  for (var i = 0; i < arr.length; ++i)
  {
    putNum(arr[i]);
  }
}

foldl([1, 2, 3, 4], newAccumulator());
document.write("Sum: " + total + "<br/>");
</script>

Answer 1

In the call to foldl you call the newAccumulator function:

foldl([1, 2, 3, 4], newAccumulator());

In the first case this returns a function that does the summing up:

return function(i) { total += i; };

In the second case the call to newAccumulator doesn’t return anything, so foldl doesn’t have a function it can call to calculate the sum.
You should pass newAccummulator directly to foldl, not the value it (doesn’t) return:

foldl([1, 2, 3, 4], newAccumulator);