function isExist(elem){

  if(typeof elem !== 'undefined' && typeof elem !== null) {

    return true;

  }else{

    return false; 

  }

}

var addCover = function(){    

  removeCover();

  var coverCss = {

    position: 'fixed',

    zIndex: '100',

    top : '0px',

    left: '0px',

    bottom: '0px',

    right: '0px',

    backgroundColor:'#000',

    opacity: 0.6

  }

  var cover = $('<div id="cover" class="mask"/>');    

  $('body').append(cover.css(coverCss));

}

var removeCover = function(){

  if(isExist($('#cover'))) $('#cover').remove();

}

​

function popup(target){

  addCover();

  target.css('position','fixed')

      .css("top",($(window).height()-target.height())/2)

      .css("left",($(window).width()-target.width())/2)

      .css("z-index","9999")

      .fadeIn();

  $(".close").on('click',function(){

    removeCover();

    target.fadeOut();

    return false;

  });

}

$('a').on('click',function(){

  popup($('.popup'))

})

​