​

html代码

<textarea id="input" type="textarea" placeholder="请输入内容" rows="3"></textarea>
<div id="num"></div>

js代码

const input = document.getElementById('input');
const num = document.getElementById('num');

input.addEventListener('input',function(){
    let len = this.value.length;
    if(len>100){
        this.value = this.value.slice(0,100);
        return false;
    }
    num.innerHTML = len; 
})

​