unload() function

unload.js

/**
 * Provide a way to remove / undo any changes made
 * either at the unloading of the script/add-on in which this function is used
 * or at the unloading of the container, if provided
 *
 * @param callback 
 *        function to call when unloading of the script or container.
 *        This function should undo all the changes made.
 * @param container
 *        Optional, the scope of the function.
 *        That is, if the scope is being unloaded, or destroyed
 *        the function would be called
 * @return removeUnloader
 *         reference to the function, which when called,
 *         prevents the function callback to be called 
 *         in case of unloading of script or container
 */
function unload(callback, container) {
  // Initialize the array of unloaders on the first usage
  let unloaders = unload.unloaders;
  if (unloaders == null)
    unloaders = unload.unloaders = [];

  // Calling with no arguments runs all the unloader callbacks
  if (callback == null) {
    unloaders.slice().forEach(function(unloader) unloader());
    unloaders.length = 0;
    return;
  }

  // The callback is bound to the lifetime of the container if we have one
  if (container != null) {
    // Remove the unloader when the container unloads
    container.addEventListener("unload", removeUnloader, false);

    // Wrap the callback to additionally remove the unload listener
    let origCallback = callback;
    callback = function() {
      container.removeEventListener("unload", removeUnloader, false);
      origCallback();
    }
  }

  // Wrap the callback in a function that ignores failures
  function unloader() {
    try {
      callback();
    }
    catch(ex) {}
  }
  unloaders.push(unloader);

  // Provide a way to remove the unloader
  function removeUnloader() {
    let index = unloaders.indexOf(unloader);
    if (index != -1)
      unloaders.splice(index, 1);
  }
  return removeUnloader;
}