原生js如何获取json字符串中某个对象的内容

有如下json字符串：

const str = `{
      input: {
        label: "输入框input",
        name: "input",
        option: {
          rules: [{ required: true, message: "Please select your country!" }]
        },
        on: {
          change: e => {
            const { value } = e.target;
            console.log("change", value);
          }
        }
      },
      number: {
        label: "数字输入框",
        name: "input",
        type: "number",
        props: {
          min: 3,
          max: 500
        },
        option: {
          rules: [{ required: true, message: "Please select your country!" }]
        },
        on: {
          change: value => {
            console.log("change", value);
          }
        }
      }
    }`;

怎样获取其中对象input或者number为字符串的内容，要得到的结果如下

获取input的结果：

{
        label: "输入框input",
        name: "input",
        option: {
          rules: [{ required: true, message: "Please select your country!" }]
        },
        on: {
          change: e => {
            const { value } = e.target;
            console.log("change", value);
          }
        }
      }

获取number的结果：

{
        label: "数字输入框",
        name: "input",
        type: "number",
        props: {
          min: 3,
          max: 500
        },
        option: {
          rules: [{ required: true, message: "Please select your country!" }]
        },
        on: {
          change: value => {
            console.log("change", value);
          }
        }
      }

原来使用 const json = JSON.parse(str); 转成json；再通过 json.input 获取需要的对象内容；然后通过JSON.stringify(json.input);转回需要的字符串；但这种方法会导致里面的 change为方法的内容获取不到。