如何限制textarea域中的字符的个数

我们都知道textarea标签没有maxlength属性，那么就需要我们用代码来进行控制，下面是针对texarea写的一个限制输入长度的

jQuery.fn.maxLength = function(max){  
            this.each(function(){  
               var type = this.tagName.toLowerCase(); 
                var inputType = this.type? this.type.toLowerCase() : null;  
                if(type == "input" && inputType == "text" || inputType == "password"){  
                    //Apply the standard maxLength  
                    this.maxLength = max;  
                }  
                else if(type == "textarea"){  
                 this.onkeypress = function(e){  
                     var ob = e || event;  
                     var keyCode = ob.keyCode;  
                     var hasSelection = document.selection? document.selection.createRange().text.length > 0 : this.selectionStart != this.selectionEnd;  
                     return !(this.value.length >= max && (keyCode > 50 || keyCode == 32 || keyCode == 0 || keyCode == 13) && !ob.ctrlKey && !ob.altKey && !hasSelection);  
                 };  
                 this.onkeyup = function(){  
                     if(this.value.length > max){  
                        this.value = this.value.substring(0,max);  
                     }  
                 };  
             }  
         });  
     };  

用法如下：

$('#mytextarea').maxLength(500);