Trying to concat multiple strings

Try the following

a!localVariables(
  local!data:{
    a!map(id:1,visitorId:11,name:"John",comment:"hii john"),
    a!map(id:2,visitorId:12,name:"Jacob",comment:"hii jacob"),
    a!map(id:3,visitorId:13,name:"David",comment:"hii david"),
    a!map(id:2,visitorId:14,name:"Bobby",comment:"hii bobby"),
    a!map(id:3,visitorId:15,name:"Mark",comment:"hii mark"),
  },
  local!distinctIds: union(
    property(local!data, "id", null),
    property(local!data, "id", null)
  ),
  a!flatten(
    a!forEach(
      items: local!distinctIds,
      expression: {
        a!localVariables(
          local!tempData: index(
            local!data,
            wherecontains(
              tointeger(fv!item),
              tointeger(property(local!data, "id", null))
            ),
            null
          ),
          a!map(
            id: fv!item,
            comment: a!forEach(
              items: local!tempData,
              expression: concat(
                property(fv!item, "name", null),
                " - ",
                property(fv!item, "comment", null)
              )
            )
          )
        )
      }
    )
  )
)

Output:

You can use joinarray to combine more than one comment.

I tried to simplify that code a bit and added a line break "char(10)" between the comments.

a!localVariables(
  local!data:{
    a!map(id:1,visitorId:11,name:"John",comment:"hii john"),
    a!map(id:2,visitorId:12,name:"Jacob",comment:"hii jacob"),
    a!map(id:3,visitorId:13,name:"David",comment:"hii david"),
    a!map(id:2,visitorId:14,name:"Bobby",comment:"hii bobby"),
    a!map(id:3,visitorId:15,name:"Mark",comment:"hii mark"),
  },
  local!distinctIds: union(local!data.id, local!data.id),
  a!forEach(
    items: local!distinctIds,
    expression: a!localVariables(
      /* Find all rows with that given id */
      local!matchingRows: index(local!data, wherecontains(fv!item, local!data.id), {}),
      a!map(
        id: fv!item,
        comment: joinarray(
          a!forEach(
            items: local!matchingRows,
            expression: concat(fv!item.name, " - ", fv!item.comment)
          ),
          char(10)
        )
      )
    )
  )
)