JSFiddle - React, Tailwind, and code Playground

HTML

<div id="divOut">
    <div id="divIn" data-foo="true">
        <div id="divIn2">
            <table>
                <tr data-foo="maybe">
                    <td>1</td>
                    <td>2</td>
                    <td>3</d>
                        <td data-foo="again">4</td>
                </tr>
            </table>    
        </div>
    </div>
</div>

JavaScript

function doMatches() {
    var matches = new Array();  
    var allDom = document.getElementsByTagName("*"); 
    for(var i =0; i < allDom.length; i++){     
        var d = allDom[i];
        if(d["data-foo"] !== undefined) {          
            matches.push(d);     
        } 
    } 
   
   alert(matches.length); 
}

doMatches();