webextensions-examples/beastify/content_scripts/beastify.js

(function() {
  /**
   * Check and set a global guard variable to
   * ensure that if this content script is injected into a page again,
   * it returns (and does nothing).
   */
  if (window.hasRun) {
    return;
  }
  window.hasRun = true;

  /**
   * Given a URL for a beast image, remove all beasts, 
   * then create and style an IMG node pointing to the image and
   * insert the node into the document.
   */
  function insertBeast(beastURL) {
    removeExistingBeasts();
    let beastImage = document.createElement("img");
    beastImage.setAttribute("src", beastURL);
    beastImage.style.objectFit = "contain";
    beastImage.style.position = "fixed";
    beastImage.style.height = "100%";
    beastImage.style.width = "100%";
    beastImage.className = "beastify-image";
    document.body.appendChild(beastImage);
  }

  /**
   * Remove all beasts from the page.
   */
  function removeExistingBeasts() {
    let existingBeasts = document.querySelectorAll(".beastify-image");
    for (let beast of existingBeasts) {
      beast.remove();
    }
  }

  /**
   * Listen for messages from the background script.
   * Depending on the message, call "beastify()" or "reset()".
   */
  browser.runtime.onMessage.addListener((message) => {
    if (message.command === "beastify") {
      insertBeast(message.beastURL);
    } else if (message.command === "reset") {
      removeExistingBeasts();
    }
  });

})();