state = {8xf0y6ziyjabvozdd253nd:[
  {
    "id": "894tuq4ut84ut8v4t8wun89g",
    "parentId": "8xf0y6ziyjabvozdd253nd",
    "timestamp": 1503045227866,
    "body": "Hi there! I ams a COMMENTSsg.",
    "author": "thingtwo",
    "voteScore": 143,
    "deleted": false,
    "parentDeleted": false
  },
]}

let index = state[payload.id].findIndex((element)=> element.id === payload.data.id)

return { ...state,
                [payload.id]:[...state[payload.id],index=payload.data]
            }

{8xf0y6ziyjabvozdd253nd:[
          {
            "id": "894tuq4ut84ut8v4t8wun89g",
            "parentId": "8xf0y6ziyjabvozdd253nd",
            "timestamp": 1503045227866,
            "body": "Hi there! I am a COMMENTS.",
            "author": "thingtwo",
            "voteScore": 143,
            "deleted": false,
            "parentDeleted": false
          },
          {
            "id": "894tuq4ut84ut8v4t8wun89g",
            "parentId": "8xf0y6ziyjabvozdd253nd",
            "timestamp": 1503045227866,
            "body": "Hi there! I am a COMMENTS Replace",
            "author": "thingtwo",
            "voteScore": 143,
            "deleted": false,
            "parentDeleted": false
          }
        ]}

{8xf0y6ziyjabvozdd253nd:[
              {
                "id": "894tuq4ut84ut8v4t8wun89g",
                "parentId": "8xf0y6ziyjabvozdd253nd",
                "timestamp": 1503045227866,
                "body": "Hi there! I am a COMMENTS Replace",
                "author": "thingtwo",
                "voteScore": 143,
                "deleted": false,
                "parentDeleted": false
              }
            ]}