Question

因此，基本上我想要实现的是在更改选择时使输入的占位符发生变化。第一个输入成功更改，但这似乎与第二个输入相同。以下是我使用的代码：＆＃13; ＆＃13; function onChange（fromOrTo）{＆＃13; if（fromOrTo =＆＃39; from＆＃39;）{＆＃13; document.getElementById（＆＃34; from＆＃34;）。placeholder = document.getElementById（＆＃34; selectFrom＆＃34;）。value;＆＃13; } else if（fromOrTo =＆＃39; to＆＃39;）{＆＃13; document.getElementById（＆＃34; to＆＃34;）。placeholder = document.getElementById（＆＃34; selectTo＆＃34;）。value;＆＃13; }＆＃13; }＆＃13; ＆LT;身体GT;＆＃13; ＆lt; div id =＆＃34; sect＆＃34;＆gt;＆＃13; ＆lt;节＆GT;＆＃13; ＆lt; select onchange =＆＃34; onChange（＆＃39; from＆＃39;）＆＃34; ID =＆＃34; selectFrom＆＃34;＆GT;＆＃13; ＆lt; option value =＆＃34; Celcius＆＃34;＆gt; Celcius＆lt; / option＆gt;＆＃13; ＆lt; option value =＆＃34; Farenheit＆＃34;＆gt; Farenheit＆lt; / option＆gt;＆＃13; ＆lt; option value =＆＃34; Kelvin＆＃34;＆gt; Kelvin＆lt; / option＆gt;＆＃13; ＆lt; option value =＆＃34; Reamur＆＃34;＆gt; Reamur＆lt; / option＆gt;＆＃13; ＆LT; /选择＆GT;＆＃13; ＆lt; span class =＆＃34; toText＆＃34;＆gt; to＆lt; / span＆gt;＆＃13; ＆lt; select onchange =＆＃34; onChange（＆＃39; to＆＃39;）＆＃34; ID =＆＃34; selectTo＆＃34;＆GT;＆＃13; ＆lt; option value =＆＃34; Celcius＆＃34;＆gt; Celcius＆lt; / option＆gt;＆＃13; ＆lt; option value =＆＃34; Farenheit＆＃34;＆gt; Farenheit＆lt; / option＆gt;＆＃13; ＆lt; option value =＆＃34; Kelvin＆＃34;＆gt; Kelvin＆lt; / option＆gt;＆＃13; ＆lt; option value =＆＃34; Reamur＆＃34;＆gt; Reamur＆lt; / option＆gt;＆＃13; ＆LT; /选择＆GT;＆＃13; ＆LT; /节＆gt;＆＃13; ＆lt;节＆GT;＆＃13; ＆lt; input type =＆＃34; text＆＃34; ID =＆＃34;从＆＃34;＆GT;＆＃13; ＆lt; span id =＆＃34; toText＆＃34;＆gt; to＆lt; / span＆gt;＆＃13; ＆lt; input type =＆＃34; text＆＃34; ID =＆＃34;至＆＃34;＆GT;＆＃13; ＆LT; /节＆gt;＆＃13; ＆LT; / DIV＆GT;＆＃13; ＆LT; /体＆GT;＆＃13; ＆＃13; ＆＃13;

Answer 1

您只需要与===而不是=

进行比较

function onChange(fromOrTo) {
  if (fromOrTo === 'from') {
    document.getElementById("from").placeholder = document.getElementById("selectFrom").value;
  } else if (fromOrTo === 'to') {
    document.getElementById("to").placeholder = document.getElementById("selectTo").value;
  }
}

<body>
  <div id="sect">
    <section>
      <select onchange="onChange('from')" id="selectFrom">
        <option value="Celcius">Celcius</option>
        <option value="Farenheit">Farenheit</option>
        <option value="Kelvin">Kelvin</option>
        <option value="Reamur">Reamur</option>
      </select>
      <span class="toText">to</span>
      <select onchange="onChange('to')" id="selectTo">
        <option value="Celcius">Celcius</option>
        <option value="Farenheit">Farenheit</option>
        <option value="Kelvin">Kelvin</option>
        <option value="Reamur">Reamur</option>
      </select>
    </section>
    <section>
      <input type="text" id="from">
      <span id="toText">to</span>
      <input type="text" id="to">
    </section>
  </div>
</body>

Answer 2

每个if条件中只有一个等号。这是变量的assigns而不是compares。切换到两个等号（或三个以避免类型强制）将解决此问题：

function onChange(fromOrTo) {
  if (fromOrTo === 'from') {
    document.getElementById("from").placeholder = document.getElementById("selectFrom").value;
  } else if (fromOrTo === 'to') {
    document.getElementById("to").placeholder = document.getElementById("selectTo").value;
  }
}

<body>
  <div id="sect">
    <section>
      <select onchange="onChange('from')" id="selectFrom">
        <option value="Celcius">Celcius</option>
        <option value="Farenheit">Farenheit</option>
        <option value="Kelvin">Kelvin</option>
        <option value="Reamur">Reamur</option>
      </select>
      <span class="toText">to</span>
      <select onchange="onChange('to')" id="selectTo">
        <option value="Celcius">Celcius</option>
        <option value="Farenheit">Farenheit</option>
        <option value="Kelvin">Kelvin</option>
        <option value="Reamur">Reamur</option>
      </select>
    </section>
    <section>
      <input type="text" id="from">
      <span id="toText">to</span>
      <input type="text" id="to">
    </section>
  </div>
</body>

输入的占位符不会从<select>更改

2 个答案: