How can I map an array with a condition

So I need to map a dN if the mD is true.
But I am not sure on how do I add the condition in order to check, the furthest I got was mapping all the dN's.

This is the payload:

[{
        "country" : "AT",
        "groupMembers": [
            {
                "dN": "268",
                
                "mD": true
            },
            {
                "dN": "240",
                
                "mD": false
            }
        ]
    },
    {
        "country" : "RO",
        "groupMembers": [
            {
                "dN": "273",
                
                "mD": true
            },
            {
                "dN": "292",
               
                "mD": false
            }
        ]
    }]

What I did was this:

payload map (value,index) -> {  country : value.country,
LC: value.groupMembers.dN }

And got this Output:

[
  { 
    "country" : "at",
    "LC": [
      "268",
      "240"
    ]
  },
  {
    "country" : "pt",
    "LC": [
      "273",
      "292"
    ]
  }]

How can I make it so that only "268" and "273" get mapped because of the true statement in "MD"?
The expected outcome should be something like :

[
  { "country" : "at",
    "LC": "268"
  },
  { "country" : "pt",
    "LC": "273"
  }
]

Below script will help you.

%dw 2.0
output application/json
---
LC : (payload.groupMembers flatMap ($ filter($.mD == true))) map (
    ($.dN)
)

One way is to map the top level elements then filter then map each list in groupMembers to only the value of mD.

%dw 2.0
output application/json
---
payload map { 
        country: $.country,
        LC:  $.groupMembers 
                filter ($.mD) 
                map ($.dN) 
    } 

Output:

[
  {
    "country": "AT",
    "LC": [
      "268"
    ]
  },
  {
    "country": "RO",
    "LC": [
      "273"
    ]
  }
]