指定秒後に、フェードアウトする その２？

var Fadeout = function ( o ) {
  this.obj = o;
  this.decrease();
  this.obj.timerId = setTimeout( (function ( that ) {
    return function ( ) { that.decrease(1); }; } )( this ), this.obj.wait );
};

Fadeout.prototype.decrease = function ( sw ) {
  var op = this.obj.opacity, f;
  if( op < 0 ) f = 1, op = 0;
  if( op > 100 ) f = 1, op = 100;
  if( f ) {
    clearInterval( this.obj.timerId );
    this.obj.endFunc && this.obj.endFunc.call( this );
    return;
  }
  this.obj.element.style./*@if (@_jscript) filter = 'alpha(opacity='+ op+ ')' @else@*/ opacity = op / 100 /*@end@*/;
  this.obj.opacity += this.obj.step;
  if( sw ) {
    this.obj.timerId = setInterval( (function (that) {
      return function () { that.decrease(); }; } )( this ) , this.obj.time);
  }
};
Fadeout.buffer = [ ];
Fadeout.add = function ( e, wait, step, time, endFunc ) {
  var i, o, f = false;
  for( i = 0; o = this.buffer[i++]; ) {
    if( o.element == e ) {
      clearTimeout( o.timerId );
      clearInterval( o.timerId );
      f = true;
      break;
    }
  }
  o = { 'element': e, 'wait': wait, 'step': step, 'time': time, 'opacity': 100, 'timerId': null, 'endFunc': endFunc };
  f || this.buffer.push( o );
  new Fadeout( o );
};

使い方
Fadeout.add( document.getElementById('xxx', 3000, -5, 30);// id, 待ち時間, 変化量, 変化時間

bufferは、同じ要素に指定がされた場合、キャンセルして新しく上書きするため