$.fn.countTo = function (options) {
 options = options || {};//当options未被初始化，即typeof options = 'undefined'时，执行后面部分即var options = {}来初始化一个对象

 return $(this).each(function () {
  // set options for current element
  var settings = $.extend({}, $.fn.countTo.defaults, {
  from:  $(this).data('from'),
  to:  $(this).data('to'),
  speed:  $(this).data('speed'),
  refreshInterval: $(this).data('refresh-interval'),
  decimals: $(this).data('decimals')
  }, options);

  // how many times to update the value, and how much to increment the value on each update
  //更新值多少次，每次更新值多快
  var loops = Math.ceil(settings.speed / settings.refreshInterval),
  increment = (settings.to - settings.from) / loops;

  // references & variables that will change with each update
  //引用和变量每次更新将改变
  var self = this,//返回html对象
  $self = $(this),//返回返回一个jquery对象
  loopCount = 0,
  value = settings.from,
  data = $self.data('countTo') || {};//获取jauery方法对象

  $self.data('countTo', data);//赋值

  // if an existing interval can be found, clear it first
  //如果存在间隔，则清除它
  if (data.interval) {
  clearInterval(data.interval);
  }
  data.interval = setInterval(updateTimer, settings.refreshInterval);

  // initialize the element with the starting value
  //用开始的值初始化
  render(value);

  function updateTimer() {
  value += increment;
  loopCount++;

  render(value);

  if (typeof(settings.onUpdate) == 'function') {
   settings.onUpdate.call(self, value);
  }

  if (loopCount >= loops) {
   // remove the interval
   $self.removeData('countTo');
   clearInterval(data.interval);
   value = settings.to;

   if (typeof(settings.onComplete) == 'function') {
   settings.onComplete.call(self, value);
   }
  }
  }

  function render(value) {
  var formattedValue = settings.formatter.call(self, value, settings);
  $self.html(formattedValue);
  }
 });
 };

 $.fn.countTo.defaults = {
 from: 200,  // the number the element should start at
 to: 0,   // the number the element should end at
 speed: 1000,  // how long it should take to count between the target numbers
 refreshInterval: 1, // how often the element should be updated
 decimals: 0,  // the number of decimal places to show
 formatter: formatter, // handler for formatting the value before rendering
 onUpdate: null, // callback method for every time the element is updated
 onComplete: null // callback method for when the element finishes updating
 };

 function formatter(value, settings) {
 return value.toFixed(settings.decimals);
 }

 // custom formatting example
 $('#count-number').data('countToOptions', {
 formatter: function (value, options) {
 return value.toFixed(options.decimals).replace(/\B(?=(?:\d{3})+(?!\d))/g, ',');
 }
 });

 // start all the timers
 $('.timer').each(count);
 function count(options) {
 var $this = $(this);
 options = $.extend({}, options || {}, $this.data('countToOptions') || {});
 $this.countTo(options);
 }