User script to resolve t.co links on twitter

resolve_tco_links.user.js

// ==UserScript==
// @name        Resolve t.co links on twitter
// @namespace   Violentmonkey Scripts
// @match       *://twitter.com/*
// @grant       GM_xmlhttpRequest
// @version     1.1
// @author      @jix_
// @homepage    https://gist.github.com/jix/e67c127820954e1d1571c1f6f800ac2c
// @description Tries to resolve all annoying t.co links on twitter and replaces them with the
//              target URL.
// ==/UserScript==


// 2021-02-23: 1.1: Also handle http not only https. Handle initial DOM nodes. Slightly improved
//                  error handling/logging.
// 2021-02-17: 1.0: Initial version

const inFlight = {}; // Have just one request in flight per URL
const cached = {}; // Cache results until page reload


// Takes a 't.co' url, strips '?amp=1' if present and resolves it by making a GET request. With curl
// on the command line you can also use a HEAD request, but I didn't manage to make it work in here.
const resolveTcoUrl = function (url, callback) {
  url = url.replace('?amp=1', '');
  if (cached.hasOwnProperty(url)) {
    callback(cached[url]);
    return;
  }
  console.log(`resolving ${url}`);

  if (inFlight.hasOwnProperty(url)) {
    // Request already in flight, just push this callback to it
    inFlight[url].push(callback);
    return;
  } else {
    inFlight[url] = [callback];
  }

  GM_xmlhttpRequest({
    method: "GET",
    url: url,
    anonymous: true,
    onload: function (response) {
      if (response.status != 200) {
        console.log(`failed resolve for ${url}`);
        // We reset the callbacks so if the url appears again we can retry... untested though
        delete inFlight[url];
        return;
      }

      let target;

      try {
        // The resulting url is in a meta http eqiv tag, easy to parse with browser APIs
        const parsed = new DOMParser().parseFromString(response.responseText, 'text/html');
        const meta = parsed.querySelector('META[http-equiv="refresh"]');
        target = meta.content.replace(/^0;URL=/, '');

        cached[url] = target; // Cache until page reload
      } catch (e) {
        console.log([`parsing response for ${url} failed`, e]);
        delete inFlight[url];
        return;
      }

      const callbacks = inFlight[url]; // Get and reset all pending callbacks
      delete inFlight[url];
      for (const callback of callbacks) {
        try {
          callback(target); // And notify them
        } catch (e) {
          console.log([`callback after resolving ${url} failed`, e]);
        }
      }
    }
  });
};

// Resolve all <a href=...> contained in the passed DOM node
const replaceContainedLinks = function (subtree) {
  for (const linkNode of subtree.querySelectorAll('a')) {
    if (!linkNode.href.startsWith('https://t.co/') && !linkNode.href.startsWith('http://t.co/')) continue;
    resolveTcoUrl(linkNode.href, function (resolved) {
      linkNode.href = resolved;
    });
  }
}

// This gets called whenever twitter javascript adds new subtrees to the DOM which we'll scan for
// links.
const mutationCallback = function (mutationsList, observer) {
  // We scan all added subtrees for all <a> tags and check whether we need to rsolve them
  for (const mutation of mutationsList) {
    for (const addedSubtree of mutation.addedNodes) {
      replaceContainedLinks(addedSubtree)
    }
  }
};

// Install the mutation callback
new MutationObserver(mutationCallback).observe(document, { childList: true, subtree: true });

// Replace links in static content
replaceContainedLinks(document);

I tried to run this code in the Firefox console today and it returned this page as an explanation for the error I received. I couldn't see where the let command in your code would interfere with anything. Do you have any insights?

This code does not run in the console but requires an extension like Violentmonkey. I also stopped using Twitter so I'm not using or maintaining this script anymore.