SOURCE

var oTxt = document.getElementById('txt');
            var oSpan = document.getElementById('span1');
            var n = 10;
            oTxt.oninput = function(){
                let num = oTxt.value.length
                if(num > n){
                    oTxt.value = oTxt.value.slice(0,n)
                }
                let low = n-oTxt.value.length
                oSpan.innerHTML = '还剩' + low + '字';
            }

<textarea id="txt" style="width:300px; height:200px;"></textarea>
    <span id="span1">还剩10字</span>