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