content.js

const createModal = element => {
  const modal = document.createElement("div");
  modal.classList.add("modal");
  const content = document.createElement("div");
  content.classList.add("modal-content");
  content.innerHTML = element;
  modal.appendChild(content);
  document.body.appendChild(modal);
};

const removeModal = () => {
  const modalElements = document.getElementsByClassName("modal");
  Array.from(modalElements).forEach(el => el.remove());
};

chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
  if (message) {
    let timeleft = 20;
    const timerFn = setInterval(() => {
      if (timeleft <= 0) {
        createModal("Well done! Your eyes thank you ʕ•́ᴥ•̀ʔっ♡");
        clearInterval(timerFn);
        setTimeout(() => {
          removeModal();
        }, 3000);
      } else {
        createModal(timeleft);
      }
      timeleft--;
    }, 1000);
  }
});