Untitled

<!DOCTYPE html>
<html lang="en">

<head>
    <title>Absent index</title>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
</head>

<body>
    <button type="submit" id="add">Add</button>
    <section></section>
</body>
<script>
    let section = document.querySelector('section');
    let elementNumbers = new Set();
    let absentElm = new Set();
    const setAttributes = (elm, attrs) => { for (let key in attrs) elm.setAttribute(key, attrs[key]) };
    const sorterSet = (oldSet) => {
        let newSet = new Set([...oldSet].sort(function (a, b) { return a - b }));
        return newSet;
    }
    const setRemover = (keys, arr) => {
        for (prop of keys) arr.delete(prop);
        return arr;
    }
    const clickSub = (e) => {
        absentElm.add(+e.target.getAttribute('elmnum'));
        document.querySelector('p#item_' + e.target.getAttribute('elmnum')).remove();
    }
    const clickAdd = () => {
        let countElements = document.querySelectorAll('p').length;
        countElements++;
        let iteratorAbsElm = sorterSet(absentElm).values();
        let absIdx = iteratorAbsElm.next().value || 0;
        let newElementNumbers = sorterSet(setRemover(absentElm, elementNumbers));
        let currentSelector = ('p#item_' + [...newElementNumbers][absIdx - 1]);
        elementNumbers.add(absIdx || countElements);
        absentElm.delete(absIdx);
        let lineP = document.createElement('p');
        let textP = document.createTextNode('Item ' + (absIdx || countElements) + ' ');
        let buttonP = document.createElement('button');
        let buttonText = document.createTextNode('Sub');
        setAttributes(buttonP, {
            'type': 'submit',
            'class': 'sub',
            'id': 'subItem_' + (absIdx || countElements),
            'elmNum': (absIdx || countElements)
        });
        buttonP.appendChild(buttonText);
        buttonP.addEventListener('click', (e) => clickSub(e))
        lineP.setAttribute('id', 'item_' + (absIdx || countElements));
        section.insertBefore(lineP, section.querySelector(currentSelector));
        lineP.appendChild(textP);
        lineP.appendChild(buttonP);
    }
    document.querySelector('button#add').addEventListener('click', clickAdd);
</script>

</html>