title: Cannot get the array in json when I put the data in the state
body: <p>I'm having trouble to get the data of array in JSON.
  I send the json file by using node.js and the code is down below.</p>
  
  <pre><code>const express = require("express");
  const router = express.Router(); // 라우터 분리
  
  router.get("/api/hello", (req, res) =&gt; {
    // app 대신 router에 연결
    res.json({ express: "hello~" });
  });
  
  router.get("/api/beef", (req, res) =&gt; {
    res.json();
  });
  
  module.exports = router; // 모듈로 만드는 부분
  </code></pre>
  
  <p>And the data which I send with json is like this.</p>
  
  <pre><code>  {
      "test": "Beef data get Completed",
      "name": "beef",
      "data": [
        { "productName": "양지", "price": 20000 },
        { "productName": "갈비", "price": 30000 },
        { "productName": "등심", "price": 15000 }
      ]
    }
  ]
  </code></pre>
  
  <p>And then I get the data by using fetch, and save it in the <strong>state</strong>.
  I get the right data which I want in the fetch function but when I try to get the data out of the fetch function I keep getting the error like <em>cannot get the property blahblahblah...</em>
  And this is the code I did in the client.</p>
  
  <pre><code>import React, { Component } from "react";
  
  const itemList = ["돼지고기", "소고기", "닭&amp;오리", "Sale", "연락처"];
  
  class Contents extends Component {
    state = {
      parsedJson : "",
    };
  
    componentDidMount() {
      this._callAPI()
        .then(parsedJson=&gt;{
          console.log("In the fetch : ",parsedJson[0].data[0])
          this.setState({
            parsedJson
          })
        })
        .catch(error =&gt; {
          console.log(error);
          this.setState({
            ...this.state,
            isError: true
          });
        });
    }
  
    _callAPI = async () =&gt; {
      const response = await fetch(`api/${this.props.curPage}`);
      const data = response.json();
      if (response.status !== 200) throw Error(data.message);
  
      return data;
    };
  
    render() {
      const stateData = this.state;
      console.log("In the render : ",stateData.parsedJson[0].data[0]) &lt;- !!!!!!!!!!!! where error occur
      return (
        &lt;&gt;
          &lt;RenderByItemList
            curPage={this.props.curPage}
            data={stateData.productData}
          /&gt;
        &lt;/&gt;
      );
    }
  }
  </code></pre>