Документ: свойство readyState

switch (document.readyState) {
  case "loading":
    // The document is loading.
    break;
  case "interactive": {
    // The document has finished loading and we can access DOM elements.
    // Sub-resources such as scripts, images, stylesheets and frames are still loading.
    const span = document.createElement("span");
    span.textContent = "A <span> element.";
    document.body.appendChild(span);
    break;
  }
  case "complete":
    // The page is fully loaded.
    console.log(
      `The first CSS rule is: ${document.styleSheets[0].cssRules[0].cssText}`,
    );
    break;
}

// Alternative to DOMContentLoaded event
document.onreadystatechange = () => {
  if (document.readyState === "interactive") {
    initApplication();
  }
};

// Alternative to load event
document.onreadystatechange = () => {
  if (document.readyState === "complete") {
    initApplication();
  }
};

document.addEventListener("readystatechange", (event) => {
  if (event.target.readyState === "interactive") {
    initLoader();
  } else if (event.target.readyState === "complete") {
    initApp();
  }
});