AWS Step Function - Adding dynamic value to Pass state type

Question

I have the following state defined in my state machine.

 "loop":{
      "Type": "Pass",
      "Result":{
        "totalCount": "$.newFieldsResponse.body.count",
        "currentCount": 0,
        "step": 1
      },
      "ResultPath": "$.iteration",
      "Next":"iterateLoop"
    },

I expect the output of the state to be:

"newFieldsResponse": {
      "isSuccess": true,
      "error": "",
      "body": {
        "count": 2,
        "fields": [...]
      }
    },
    "iteration": {
      "totalCount": 5,
      "currentCount": 0,
      "step": 1
    }
  }

iteration property is added to the input with totalCount property to be set to count of items in fields array.

However, the output for "iteration" property is set as:

"iteration": {
      "totalCount": "$.newFieldsResponse.body.count",
      "currentCount": 0,
      "step": 1
    }

It looks like the value "$.newFieldsResponse.body.count" is not getting resolved and is output as is.

Is there something I am doing wrong ? Can someone please advice on how to make it work ?

Answer 1

This is possible through "Parameters" in pass state

JSON

{
  "loop": {
    "Type": "Pass",
    "Parameters": {
      "totalCount": "$.newFieldsResponse.body.count",
      "currentCount": 0,
      "step": 1
    },
    "ResultPath": "$.iteration",
    "Next": "iterateLoop"
  }
}