if index is set

with(
    local!startChart: {
        1,
        2,
        3,
        4
    },
    local!chartIndex: 2,

    if (
        local!chartIndex > count(local!startChart), 
        0, 
        index(local!startChart, local!chartIndex, 0)
    )
)

I think the index() function will return null if it is not set. But you can replace the last parameter, using 0 in order to return zero if the index is not set.

Additionally, you can perform a check before using the value for chartIndex if it is within the bounds of the number of elements of the local!startChart.

Using the array notation with "variable[index]" format is also not a good practice because the index might be invalid. 

with(
    local!startChart: {
        1,
        2,
        3,
        4
    },
    local!chartIndex: 2,

    if (
        local!chartIndex > count(local!startChart), 
        0, 
        index(local!startChart, local!chartIndex, 0)
    )
)

I think the index() function will return null if it is not set. But you can replace the last parameter, using 0 in order to return zero if the index is not set.

Additionally, you can perform a check before using the value for chartIndex if it is within the bounds of the number of elements of the local!startChart.

Using the array notation with "variable[index]" format is also not a good practice because the index might be invalid. 

I think it's simpler than this. fn!index() will look for an item at the index provided and, if not found, can return a default value:

a!localVariables(
  local!myArray: {1,2,3,4,5},
  fn!index(local!myArray, 7, 99)
)

In this example, I have an array of 5 elements, and I'm looking for the value in the 7th position - which doesn't exist - so I return 99 instead.

Or have I missed some nuance here?