JSONata, how to transform array with elements missing?

Question

I am trying to convert an 2D array, let's say rows, into an array of the same rows, but for some rows one attribute is missing (omitted) and needs to be taken from one of the previous rows for which the attribute was filled.

Unfortunately, I cannot find a way to access the previous rows or to store the attribute in a variable that is persistent.

I hope somebody can give me a hint, how this is best achieved.

Let's assume the data looks like this:

{
  "payload": [
    {
      "Name": "Name1",
      "Values": "MsgId1.3 / Type1 / "
    },
    {
      "Values": "MsgId1.1 / Type3 / COMP"
    },
    {
      "Name": "Name2",
      "Values": "MsgId2.6 / Type1 / COMP"
    },
    {
      "Values": "MsgId2.5 / Type4 / COMP"
    },
    {
      "Values": "MsgId2.4 / Type4 / REJT"
    },
    {
      "Name": "Name3",
      "Values": "MsgId3.2 / Type7 / "
    }
  ]
}

The expected result looks like this:

{
  "list": [
    {
      "NAME": "Name1",
      "MSG_ID": "MsgId1.3",
      "MSG_TYPE": "Type1",
      "MSG_STATUS": ""
    },
    {
      "NAME": "Name1",   /* <-- this line is missing in my results */
      "MSG_ID": "MsgId1.1",
      "MSG_TYPE": "Type3",
      "MSG_STATUS": "COMP"
    },
    {
      "NAME": "Name2",
      "MSG_ID": "MsgId2.6",
      "MSG_TYPE": "Type1",
      "MSG_STATUS": "COMP"
    },
    {
      "NAME": "Name2",   /* <-- this line is missing in my results */
      "MSG_ID": "MsgId2.5",
      "MSG_TYPE": "Type4",
      "MSG_STATUS": "COMP"
    },
    {
      "NAME": "Name2",   /* <-- this line is missing in my results */
      "MSG_ID": "MsgId2.4",
      "MSG_TYPE": "Type4",
      "MSG_STATUS": "REJT"
    },
    {
      "NAME": "Name3",
      "MSG_ID": "MsgId3.2",
      "MSG_TYPE": "Type7",
      "MSG_STATUS": ""
    }
  ]
}

My last JSONata is like this, but it does not work.

{
  "list": [
    $.payload
      .(
        $values := $.Values ~> $split(" / ");
        $name := ( ($.Name) ? ($.Name) : $name );
        {
          "NAME": $name,
          "MSG_ID": $values[0],
          "MSG_TYPE": $values[1],
          "MSG_STATUS": $values[2]
        }
      )
  ]
}

I also tried with $each() function, but to no avail.

Answer 1

Andrew Coleman's answer put me in the right direction.

My final solution can deal with large data sets. It runs quick even with thousands of objects in the payload array.

Example with solution can be found here.

(
  $previous := function($index) {
    ($index > 0) ?
      ($.payload[$index - 1].Name ?
        $.payload[$index - 1].Name :
        $previous($index - 1)) : 
      "" };

  {
    "list": $
      .payload#$i
      .(
        $values := $.Values ~> $split(" / ");
        {
          "pos": (0) ? $i,
          "NAME": ($.Name) ? ($.Name) : $previous($i),
          "MSG_ID": $values[0],
          "MSG_TYPE": $values[1],
          "MSG_STATUS": $values[2]
        }
      )
  }
)

.payload#$i uses Positional Variable Binding (#$i) to give me the index of the row of the original array payload.

"NAME": ($.Name) ? ($.Name) : $previous($i), checks if the Name is filled in the current row, if not a previous Name is searched via function $previous.

Function $previous checks if the Name of the previous row is filled and returns it. Otherwise it calls itself recursively until it finds the row above with Name filled.