How to Identify last element in an Array in Dataweave - mule

Question

i have a dataweave expression to create a custom String , but i do not want to append || to the last element of the input array . How can i achieve it ?
%dw 2.0
output application/json
var arr = ["ABC","XYZ"]
---
arr map((item,index) -> {
index: "custom ('" ++ item ++ "'); " ++ "||"
})

Answer 1

Would this help?
Script
%dw 2.0
output application/json
var arr = ["ABC","XYZ"]
---
arr map((item,index) -> {
index: if((index) < (sizeOf(arr)-1)) "custom ('" ++ item ++ "'); " ++ "||" else item
})
Output
[
{
"index": "custom ('ABC'); ||"
},
{
"index": "XYZ"
}
]

Answer 2

You could also try with the following:
%dw 2.0
output application/json
import * from dw::core::Arrays
var arr = ["ABC","XYZ"]
---
take(arr,(sizeOf(arr)-1)) map (
$ ++ "||" // do your manipulation to modify the custom entry
)
take is documented here

Answer 3

Out of curiosity, are you then taking this resulting array and joining it into a string? Your question seems to indicate the end goal of the function is returning a string. If you are, you can just map and then joinBy
%dw 2.0
output application/json
var arr = ["ABC","XYZ", "DKJL"]
---
arr map "custom($($));" joinBy " || "
Outputs
"custom(ABC); || custom(XYZ); || custom(DKJL);"
And if like in Salim's answer you shouldn't be wrapping the last value in custom, you could also take advantage of the default behavior of reduce:
%dw 2.0
output application/json
var arr = ["ABC","XYZ", "DKJL"]
---
arr[-1 to 0] reduce ("custom($($)); || " ++ $$)
Outputs
"custom(ABC); || custom(XYZ); || DKJL"