What is the best way to iterate through an array in Classic Asp VBScript?

Why not use For Each? That way you don't need to care what the LBound and UBound are.

Dim x, y, z
x = Array(1, 2, 3)

For Each y In x
    z = DoSomethingWith(y)
Next

There is a good reason NOT TO USE For i = LBound(arr) To UBound(arr)

dim arr(10) allocates eleven members of the array, 0 through 10 (assuming the VB6 default Option Base).

Many VB6 programmers assume the array is one-based, and never use the allocated arr(0). We can remove a potential source of bug by using For i = 1 To UBound(arr) or For i = 0 To UBound(arr), because then it is clear whether arr(0) is being used.

For each makes a copy of each array element, rather than a pointer.

This has two problems.

1. When we try to assign a value to an array element, it doesn't reflect on original. This code assigns a value of 47 to the variable i, but does not affect the elements of arr.

   arr = Array(3,4,8)
   for each i in arr
        i = 47
   next i
   Response.Write arr(0) '- returns 3, not 47

2. We don't know the index of an array element in for each, and we are not guaranteed the sequence of elements (although it seems to be in order).

LBound may not always be 0.

Whilst it is not possible to create an array that has anything other than a 0 Lower bound in VBScript, it is still possible to retrieve an array of variants from a COM component which may have specified a different LBound.

That said I've never come across one that has done anything like that.