Forces the highest quality video on Reddit

redditvideoquality.user.js

// ==UserScript==
// @name         Force highest resolution on Reddit
// @namespace    http://tampermonkey.net/
// @version      0.2
// @description  Parses the mpd file sent to reddit's video player and removes all but the highest resolution representations for each video period
// @author       x0a
// @run-at       document-start
// @match        https://www.reddit.com/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=reddit.com
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    const origOpen = XMLHttpRequest.prototype.open;
    const origSend = XMLHttpRequest.prototype.send;

    const highestResOnly = xml => {
        const periods = Array.from(xml.querySelectorAll('Period'));

        // for each period, find the highest bandwidth representation and cut out all the rest.
        periods.forEach(period => {
            const representations = period.querySelectorAll('AdaptationSet[contentType="video"] Representation');
            const highest = Array.from(representations)
            .map(el => ~~el.getAttribute('bandwidth'))
            .reduce((highest, current) => current > highest ? current : highest, 0);

            representations.forEach(representation => {
                if(representation.getAttribute('bandwidth') !== '' + highest){
                    representation.remove(); // remove representations that dont match the highest bandwidth
                }
            })
        })
        return xml
    }
    const highestResOnlyText = string => {
        const parser = new DOMParser();
        const xml = parser.parseFromString(string, "text/xml");
        const fixed = highestResOnly(xml);
        return fixed.documentElement.outerHTML;
    }

    XMLHttpRequest.prototype.open = function () {
        const [method, url, async, user, password] = arguments;

        if(url.indexOf('/DASHPlaylist.mpd?') !== -1){
            console.log('Found', url, this);
            this.overrideSend = true;
        }
        return origOpen.apply(this, [method,
                                     url,
                                     async === undefined ? true : async,
                                     user,
                                     password]);
    }

    XMLHttpRequest.prototype.send = function () {
        if(this.overrideSend){
            const self = this;
            const lastLoad = self.onload;
            const lastLoadEnd = self.onloadend;
            let modified = false;

            const hookModifications = () => {
                // since we happen to know that reddit doesn't set the XMLHttpRequest.responseType property on this request, I'm not bothering to handle responseText/response differently. they are both strings.
                // similarly, reddit appears to use the onload event first rather than onloadend, so I'm not bothering to handle these seperately.
                // but the code is there in case this changes.

                if(modified) return;
                
                const responseText = highestResOnlyText(self.responseText);
                const response = responseText;
                
                if(self.responseXML) {
                    try{
                        highestResOnly(self.responseXML);
                    }catch(e){
                        console.error('Could not parse response XML', e);
                    }
                }
                Object.defineProperties(self, {
                    response: {
                        get: function() {
                            console.error('Someone just tried to access me, check stack trace if you want to dig deeper');
                            return response;
                        }
                    },
                    responseText: {
                        get: function() {
                            console.error('Someone just tried to access me');
                            return responseText;
                        }
                    }

                });

                modified = true;
            }
            self.onload = function(){
                hookModifications();
                return lastLoad.apply(this, arguments);
            }

            self.onloadend = function(){
                hookModifications(); // in theory, this is already done but meh.
                return lastLoadEnd.apply(this, arguments);
            }
        }


        return origSend.apply(this, arguments);
    }

})();