background.js

const showHoursCount = () => {
    const arrayMethods = Object.getOwnPropertyNames( Array.prototype );
    const attachArrayMethodsToNodeList = (methodName) => {
      if(methodName !== "length") {
        NodeList.prototype[methodName] = Array.prototype[methodName];
      }
    };
    arrayMethods.forEach((item) => attachArrayMethodsToNodeList(item));

    const listCards = document.querySelectorAll('.list-cards');
    const estimateLabels = listCards.map((list) => list.querySelectorAll('.card-label-yellow'));
    const actualLabels = listCards.map((list) => list.querySelectorAll('.card-label-green'));

    const getHourCount = (labels) => labels.map((label) => {
        const labelsArray = label.map((item) => parseFloat(item.innerHTML));
        return labelsArray.length === 0 ? 0 :
            labelsArray.reduce((pre, next) => pre + next);
    });

    const estimateTimeCounts = getHourCount(estimateLabels);
    const actualTimeCounts = getHourCount(actualLabels);

    listCards.map((item, i) => {
        const countContainer = document.createElement('div');
        countContainer.className = 'time-label-count';
        countContainer.innerHTML = `<span class="actual-time">${actualTimeCounts[i]}</span><span class="estimate-time">${estimateTimeCounts[i]}</span>`;

        if (!!item) {
            const oldCountContainer = item.previousSibling;
            if (oldCountContainer.className === 'time-label-count') {
                item.parentNode.replaceChild(countContainer, oldCountContainer);
            } else {
                item.parentNode.insertBefore(countContainer, item);
            }
        }
    });
};

window.onload = () => showHoursCount();

setInterval(showHoursCount, 3000);