checked maxLength

 	function isMaxLen(o){  
 			var nMaxLen=o.getAttribute?  parseInt(o.getAttribute("maxlength")):"";  
 			if(o.getAttribute && o.value.length>nMaxLen){  
 				o.value=o.value.substring(0,nMaxLen)  
 			}
 		}
<input type="text" onkeyup="return isMaxLen(this)" />