Text box (instead of input) input length limit, suggesting

<div class="inform_content_text">
            <textarea name="name" placeholder="其他理由..." rows="8" cols="80" id="txt"></textarea>
            <p class="textNumberLength" style="text-align:right;">100字</p>
          </div>

          textarea{
            width: 100%;
            height: 6rem;
          }
          .inform_content_text{
            width: 100%;
            padding: 0 .5rem;
            background: #fff;
            margin-top: .25rem;
          }

        var maxLen = 100;
        var remLen = document.querySelector('.textNumberLength');
        function checkMaxInput(obj){
          if(obj.value.length>maxLen){
            obj.value=obj.value.substring(0,maxLen);
            remLen.innerHTML='超出字数限制'
          }
          else{
            remLen.innerText=(maxLen-obj.value.length)+'字';
          }
        }
        window.onload=function(){
          var otxt=document.getElementById("txt");
          otxt.onkeyup=function(){
            checkMaxInput(this);
          }
        }