为什么document.getElementById输入类型=文本失败

Question

Im trying to get data from form using getElementById. It works if i get from select form(like below), but why it fails when i try to get from textbox?

<select name="permission" id="permission" class="select2">
    <option value="rw">Read &amp; Write</option>
    <option value="r">Read Only</option>
</select>
<p id="demo"></p>

<script>
    var x= document.getElementById("permission").value;
    document.getElementById("demo").innerHTML = x;
</script>

If I change "permission" to "tenants" to get from below textbox, it doesn't show. I am confused ._.

<input type="text" id="tenants" name="tenants" class="form-control">

Answer 1

<input type="text" id="tenants" name="tenants" class="form-control">

If you want to get a value from this above code, use this.

var x = document.getElementById("tenants").value;

Looking at your original script

<select name="permission" id="permission" class="select2">
    <option value="rw">Read &amp; Write</option>
    <option value="r">Read Only</option>
</select>
<p id="demo"></p>
<script>
  var x= document.getElementById("permission").value;
  document.getElementById("demo").innerHTML = x;
</script>

This code only gets the value from select with an id permission and store it to p with an id demo.