Javascript 为什么即使我指定了值，它仍返回null？

下面的代码返回null，即使我指定了一个特定的值。我认为问题在于

document.getelementbyid（'userInput'）

显然返回null，但我认为这不是100%的情况。 谢谢

从“React”导入React
从“react static”导入{WithRoutedData，Link}
//
导出默认WithRoutedData（（{Currences}）=>(
角隅
克里普托斯
代币


加密/令牌
最小取款数量
最小订单数量
{currency.filter（currency=>currency.currency===document.getElementById（'userInput'）.value）.map（currency=>(
{currency.currency}
{货币.最低提款额}
{货币.最小订单数量}
))}
))

您可以尝试使用ref而不是document.getElementById来获取值

export default withRouteData(({ currencies }) => (
<div> 

  <Link to="/">Quoinex</Link>
  <Link to="/qryptos"><b>Qryptos</b></Link>
  <form id="form">
    <input type="text" name="name" placeholder="BTC etc." id="userInput" ref="userInput" value="BTC" />
    <input className="sub" type="submit" />
  </form>
  <h1>Tokens</h1>
  <br />
  <table className="myTable">
    <tr>
      <th>Crypto/Token</th>
      <th>Min Withdrawal Qty</th>
      <th>Min Order Qty</th>
    </tr>
    { currencies.filter(currency => currency.currency === React.findDOMNode(this.refs.userInput).value).map(currency => (
      <tr key={currency.currency}>
        <td id="tokenName">{currency.currency}</td>
        <td>{currency.minimum_withdrawal}</td>
        <td>{currency.minimum_order_quantity}</td> 
      </tr>
    ))}
  </table>

</div>
))

export default with routedData（（{currencies}）=>(
角隅
克里普托斯
代币


加密/令牌
最小取款数量
最小订单数量
{currency.filter（currency=>currency.currency===React.findDOMNode（this.refs.userInput.value）.map（currency=>(
{currency.currency}
{货币.最低提款额}
{货币.最小订单数量}
))}
))

export default withRouteData(({ currencies }) => (
<div> 

  <Link to="/">Quoinex</Link>
  <Link to="/qryptos"><b>Qryptos</b></Link>
  <form id="form">
    <input type="text" name="name" placeholder="BTC etc." id="userInput" ref="userInput" value="BTC" />
    <input className="sub" type="submit" />
  </form>
  <h1>Tokens</h1>
  <br />
  <table className="myTable">
    <tr>
      <th>Crypto/Token</th>
      <th>Min Withdrawal Qty</th>
      <th>Min Order Qty</th>
    </tr>
    { currencies.filter(currency => currency.currency === React.findDOMNode(this.refs.userInput).value).map(currency => (
      <tr key={currency.currency}>
        <td id="tokenName">{currency.currency}</td>
        <td>{currency.minimum_withdrawal}</td>
        <td>{currency.minimum_order_quantity}</td> 
      </tr>
    ))}
  </table>

</div>
))