Edit in JSFiddle

jQuery.fn.center = function(parent) {
    if (parent) {
        parent = this.parent();
    } else {
        parent = window;
    }
    this.css({
        "position": "absolute",
        "top": ((($(parent).height() - this.outerHeight()) / 2) + $(parent).scrollTop() + "px"),
        "left": ((($(parent).width() - this.outerWidth()) / 2) + $(parent).scrollLeft() + "px")
    });
return this;
}
$("div.target:nth-child(1)").center(true);
$("div.target:nth-child(2)").center(false);
$(window).resize(function() {
	$("div.target:nth-child(1)").center(true);
	$("div.target:nth-child(2)").center(false);
});

<div class="container-demo-jquery">
    <div class="target">1<br>родитель</div>
    <div class="target">2<br>окно<br/>браузера</div>
</div>

div.container-demo-jquery{
    width: 300px;
    height: 300px;
    border: 1px solid blue;
    position: relative;
    top: 10px;
    left: 10px;
}

div.target{
    width:100px;
    height:100px;
    color:white;
    background: blue;
    text-align:center;
}