dx表达式怎样取数组的最后一项内容 about ncform HOT 3 CLOSED

{
  "type": "object",
  "properties": {
    "firstname": {
      "type": "array",
      "value": [{"name": "a"}, {"name": "b"}],
      "items": {
        "type": "object",
        "properties": {
          "name": {
            "type": "string"
          }
        }
      },
      "ui": {
        "columns": 6
      }
    },
    "lastname": {
      "type": "string",
      "ui": {
        "columns": 6
      }
    },
    "fullname": {
      "type": "string",
      "valueTemplate": "dx: {{$root.firstname}}[{{$root.firstname.length}} - 1]['name']",
      "ui": {
        "columns": 12
      }
    }
  }
}