Jquery插件判断文本框还剩多少字符可以输入

javascript

function $(id) {  
    return document.getElementById(id);  
}  
  
var maxLen=255;  
function checkMaxInput(){  
    if($("summary").value.length>maxLen){  
        $("summary").value=$("summary").value.substring(0,maxLen);  
    }else{  
        $("leaves").innerHTML=maxLen-$("summary").value.length;  
    }  
}  

<tr>  
  <td>摘要：</td>  
  <td>  
    <html:textarea  property="summary"  rows="5" cols="60"  onkeyup="checkMaxInput()"/>  
    <br>  
     还可以输入<span class="red" id="leaves">255</span>个字符  
   </td>  
</tr>  


jQuery插件textlimit实现Javascript统计和限制字符个数功能 

http://www.biuuu.com/p870.html 

使用jQuery插件textlimit可以实现统计和限制字符个数功能，可应用于文本框与文本区域， 
当输入文字时textlimit插件会及时统计当前文本框与文本区域中的字符个数，如果达到限制数则不允许输入，同时可设置字符删除速度， 

使用实例 
一，包含文件部分 

   1. <script type="text/javascript" src="jquery.js"></script> 
   2. <script type="text/javascript" src="textlimit.js"></script> 

二，HTML部分 

   1. <input type="text" name="test" value="" id="test" /><span>20</span>/256 

三，Javascript部分 

   1. <script type="text/javascript"> 
   2. jQuery(document).ready(function(){ 
   3. jQuery("#test").textlimit("span",256); 
   4. }); 
   5. </script> 

当在ID为test的文本框中输入文字时，textlimit插件统计当前输入字符个数并显示在一个span的元素中，如上效果图，textlimit接口如下： 

   1. textlimit(counter_el, thelimit, speed) 

接口参数说明： 
counter_el表示显示当前统计个数的选择器标签，如：span 
thelimit表示限制个数，也就是最多可输入的个数，如：256 
speed表示删除字符速度，默认为15，注意，如果不需要可设置为-1，但不能是0 

注意：英文字符与汉字字符都统计为一个字符 

textlimit插件统计和限制字符数非常简单，具体大家可以看看textlimit的库文件，非常值得推荐。 

/* 
 * TextLimit - jQuery plugin for counting and limiting characters for input and textarea fields 
 *  
 * pass '-1' as speed if you don't want the char-deletion effect. (don't just put 0) 
 * Example: jQuery("Textarea").textlimit('span.counter',256) 
 * 
 * $Version: 2009.07.25 +r2 
 * Copyright (c) 2009 Yair Even-Or 
 * vsync.design@gmail.com 
*/  
(function(jQuery) {  
    jQuery.fn.textlimit=function(counter_el, thelimit, speed) {  
        var charDelSpeed = speed || 15;  
        var toggleCharDel = speed != -1;  
        var toggleTrim = true;  
        var that = this[0];  
        var isCtrl = false;   
        updateCounter();  
          
        function updateCounter(){  
            if(typeof that == "object")  
                jQuery(counter_el).text(thelimit - that.value.length+" characters remaining");  
        };  
          
        this.keydown (function(e){   
            if(e.which == 17) isCtrl = true;  
            var ctrl_a = (e.which == 65 && isCtrl == true) ? true : false; // detect and allow CTRL + A selects all.  
            var ctrl_v = (e.which == 86 && isCtrl == true) ? true : false; // detect and allow CTRL + V paste.  
            // 8 is 'backspace' and 46 is 'delete'  
            if( this.value.length >= thelimit && e.which != '8' && e.which != '46' && ctrl_a == false && ctrl_v == false)  
                e.preventDefault();  
        })  
        .keyup (function(e){  
            updateCounter();  
            if(e.which == 17)  
                isCtrl=false;  
  
            if( this.value.length >= thelimit && toggleTrim ){  
                if(toggleCharDel){  
                    // first, trim the text a bit so the char trimming won't take forever  
                    // Also check if there are more than 10 extra chars, then trim. just in case.  
                    if ( (this.value.length - thelimit) > 10 )  
                        that.value = that.value.substr(0,thelimit+100);  
                    var init = setInterval  
                        (   
                            function(){   
                                if( that.value.length <= thelimit ){  
                                    init = clearInterval(init); updateCounter()   
                                }  
                                else{  
                                    // deleting extra chars (one by one)  
                                    that.value = that.value.substring(0,that.value.length-1); jQuery(counter_el).text('Trimming... '+(thelimit - that.value.length));  
                                }  
                            } ,charDelSpeed   
                        );  
                }  
                else this.value = that.value.substr(0,thelimit);  
            }  
        });  
          
    };  
})(jQuery);