《JavaScript 源码分析》之 maxlength.js

 /*
   @author: Terry
   @params:
     feedback - the selector for the element that gives the user feedback. Note that this will be relative to the form the plugin is run against.
     hardLimit - whether to stop the user being able to keep adding characters. Defaults to true.
     useInput - whether to look for a hidden input named 'maxlength' instead of the maxlength attribute. Defaults to false.
     words - limit by characters or words, set this to true to limit by words. Defaults to false.
   @license:
   @version: 1.2
   @changes: code tidy via Ariel Flesler and fix when pasting over limit and including \t or \n
 */

 (function ($) {

 $.fn.maxlength = function (settings) {

     if (typeof settings == 'string') {
         settings = { feedback : settings };
     }

     settings = $.extend({}, $.fn.maxlength.defaults, settings);

     function length(el) {
         var parts = el.value;
         if ( settings.words )
             parts = el.value.length ? parts.split(/\s+/) : { length : 0 };
         return parts.length;
     }

     return this.each(function () {
         var field = this;
         var    $field = $(field);
         var $form = $(field.form);
         var    limit = settings.useInput ? $form.find('input[name=maxlength]').val() : $field.attr('maxlength');
         var    $charsLeft = $form.find(settings.feedback);

         function limitCheck(event) {
             var len = length(this);
             var exceeded = len >= limit;
             var code = event.keyCode;
             if (!exceeded )
                 return;

                 switch (code) {
                     case 8:  // allow delete  BackSpace BackSpace
                     case 9:  // Tab Tab
                     case 17: // Next
                     case 36: // Home
                     case 35: // End
                     case 37: //
                     case 38: //Up Arrow
                     case 39: //Right Arrow
                     case 40: //Dw Arrow
                     case 46: //~
                     case 65: //A
                         return;

                     default:
                         return settings.words && code != 32 && code != 13 && len == limit;
                 }
         }

         var updateCount = function () {
             var len = length(field);
             var diff = limit - len;

             $charsLeft.html( diff || "0" );

             // truncation code
             if (settings.hardLimit && diff < 0) {
                 field.value = settings.words ?
                     // split by white space, capturing it in the result, then glue them back
                     field.value.split(/(\s+)/, (limit*2)-1).join('') :
                     field.value.substr(0, limit);

                 updateCount();
             }
         };
         $field.keyup(updateCount).change(updateCount);
         if (settings.hardLimit) {
             $field.keydown(limitCheck);
         }

         updateCount();
     });
 };

 $.fn.maxlength.defaults = {
     useInput : false,
     hardLimit : true,
     feedback : '.charsLeft',
     words : false
 };

 })(jQuery);