輸入多少字 還剩多少字

//---------當前輸入多少字  還能輸入多少字------------這裏有一個bug 用戶粘貼會出錯
var textarea = fm.motto,
     curLenSpan =document.getElementById('curLen'),
     maxLenSpan =document.getElementById('maxLen'),
     leftLenSpan =document.getElementById('leftLen'),
     maxLen = +maxLenSpan.innerHTML,
    textareaLimit = document.getElementById('textareaLimit'),
     curLen = textarea.value.length,
     leftLen = maxLen - curLen;


     curLenSpan.innerHTML =curLen ;
     leftLenSpan.innerHTML = leftLen;
    textareaLimit.style.display='inline';

    textarea.onkeyup=function () {
        curLen=this.value.length;
        if(curLen>=maxLen){
            this.value=this.value.substring(0,maxLen-1);
            curLen=maxLen;
        }
        leftLen = maxLen-curLen;
        curLenSpan.innerHTML =curLen ;
        leftLenSpan.innerHTML = leftLen;
    };