JSFiddle - React, Tailwind, and code Playground

HTML

<div id="outter">
    <div id="inner">
        <div id="alone">
            Display console to show output
        </div>
    </div>
</div>

JavaScript

var firstNodelist = document.querySelectorAll("#outter, #inner");
var finalArray = new Array();

for (var i = 0; i < firstNodelist.length; i++) {

    var secondNodelist = firstNodelist[i].querySelectorAll("div");

    var firstArray = new Array();

    for (var x = 0; x < secondNodelist.length; x++) {

        firstArray.push(secondNodelist[x]);

    }

    finalArray = finalArray.concat(firstArray)

}

console.log("FINAL", finalArray);