Jawaban test code:

frontend-developer-jds.js

function appendChildren(decorateDivFunction) {
    var allDivs = document.getElementsByTagName("div");
    divlength = allDivs.length;
    for (var i = 0; i < divlength; i++) {
        var newDiv = document.createElement("div");
        decorateDivFunction(newDiv);
        allDivs[i].appendChild(newDiv);
    }
}

// Example case. 
document.body.innerHTML = `
<div id="a">
    <div id="b">
    </div>
</div>`;

appendChildren(function(div) {});
console.log(document.body.innerHTML);

test.html

<!-- jawaban langsung dari browser, diubah sedikit dengan teknik beautifier agar mudah dibaca -->
<!-- test dengan 2 level html -->
<div id="a">
    <div id="b">
        <div></div>
    </div>
    <div></div>
</div>
<!-- test dengan 5 level html -->
<div id="a">
        <div id="b">
            <div id="c">
                <div id="d">
                    <div id="e">
                        <div></div>
                    </div>
                <div></div>
            </div>
            <div></div>
        </div>
        <div></div>
    </div>
    <div></div>
</div>