demand

<select>Drop down to select the content,<input>The corresponding content is displayed in the input box.

implementation

<! DOCTYPEhtml>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="Width = device - width, initial - scale = 1.0">
  <title>Document</title>
</head>

<body>
  <table border="1px" style="width:530px; height: 320px; border-style: groove">
    <tr style="height: 40px;">
      <td style="width: 170px;">
        <center>Please fill in the account application form below</center>
      </td>
      <td bgcolor="#99ccff"></td>
    </tr>
    <tr style="height: 40px;">
      <td style="width: 170px; text-align:right">Class level:</td>
      <td>
        <select id="banji" onchange="setXuehao(this.value)" style="width: 160px; height: 35px;">
          <option selected>Please select a class</option>
          <option value="xinguan171">Fuse 171</option>
          <option value="xinguan172">Fuse 172</option>
        </select>
      </td>
    </tr>
    <tr style="height: 40px;">
      <td style="width: 170px; text-align:right">Study no. :</td>
      <td>
        <input type="text" id="xuehao">
        <span style="color: red">*</span>
        <span>Note: Only student id can be entered.</span>
      </td>
    </tr>
  </table>

  <script>
    function setXuehao() {
      // var selectidValue = document.getElementById('banji').options[document.getElementById('banji').selectedIndex].value;
      var selectidValue = document.getElementById('banji').options[document.getElementById('banji').selectedIndex].value;
      var inputValue = document.getElementById('xuehao').value;
      // if(document.getElementById('banji').selectedIndex ! = 0)
      // document.getElementById('xuehao').value="2017102341"+document.getElementById('banji').selectedIndex;
      if (selectidValue == "xinguan171")
        document.getElementById('xuehao').value = "2017102341";
      else if (selectidValue == "xinguan172")
        document.getElementById('xuehao').value = "2017102342";
      else
        document.getElementById('xuehao').value = "";
    }
  </script>

</body>

</html>
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