Assign the obtained text element to var x; why does it not take effect to determine whether the input text is empty? document.getElementById("demo").innerHTML; Is the element obtained a string or something else? It is a string. Why does using x.length to determine the length of a string not work?

<style>
input {border:1px solid #ddd;}
</style>
<p>
<form>
<label>Please Enter nickname:</label><input type="text" name="fname" id="demo"><button onclick="infoBtn()">Submit information</button>
</form>
<script>
function infoBtn(){

   var x=document.getElementById("demo").innerHTML;
   if(x.length == 0 ||x==""){
       alert("输入不能为空");
       }else{
            alert("设置成功");
           }
   }

</script>
</p>