JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <strong>These are p tags, numbered 1 - 22.</strong>
    <br/>
     <br/> <br/>
    <p>1</p>
    <p>2</p>
    <p>3</p>
    <p>4</p>
    <p>5</p>
    <p>6</p>
    <p>7</p>
    <p>8</p>
    <p>9</p>
    <p>10</p>
    <p>11</p>
    <p>12</p>
    <p>13</p>
    <p>14</p>
    <p>15</p>
    <p>16</p>
    <p>17</p>
    <p>18</p>
    <p>19</p>
    <p>20</p>
    <p>21</p>
    <p>22</p>
</div>

CSS

#container {
    color: white;
 background-color: blue;
    height: 400px;
    overflow: hidden;
}

JavaScript

var h = $("body").height();
var hiddenEls = new Array();

$("#container").find("*").each(function(){
    if ($(this).position().top > h)
        hiddenEls.push($(this));
});

alert (hiddenEls.length);