How to execute a user script before a page is saved

1. Enable the hidden option userScriptEnabled by exporting the settings from the options page, editing the JSON file, replacing userScriptEnabled: false with userScriptEnabled: true, and importing the modified file in SingleFile.

2. Dispatch the custom event single-file-user-script-init in the user script.

dispatchEvent(new CustomEvent("single-file-user-script-init"));

3. Listen to the custom event single-file-on-capture-request in the user script. The listener function is called just before the page is saved.

addEventListener("single-file-on-capture-request", event => {
  console.log("The page will be saved by SingleFile");
});

If the listener function has to be async, call event.preventDefault() and dispatch the custom event single-file-on-capture-response once the asynchronous tasks are terminated.

addEventListener("single-file-on-capture-request", async event => {
  event.preventDefault();
  try {
    await ...
  } finally {
    dispatchEvent(new CustomEvent("single-file-on-capture-response"));
  }
});

4. The complete example below shows how to remove images from the page just before saving it.

// ==UserScript==
// @name         Remove images
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Remove all the images
// @author       Gildas Lormeau
// @match        *://*/*
// @grant        none
// ==/UserScript==

(() => {
    dispatchEvent(new CustomEvent("single-file-user-script-init"));
    addEventListener("single-file-on-capture-request", event => {
        Array.from(document.images).forEach(image => {
            image.setAttribute("src", "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8/5+hHgAHggJ/PchI7wAAAABJRU5ErkJggg==")
            image.removeAttribute("data-src");
            image.removeAttribute("data-srcset");
            image.removeAttribute("srcset");
            image.removeAttribute("data-single-file-lazy-loaded-src");
        });
    });
})();