injectedUdemy.js

let video;
let targetVideoSpeed = 1;

function urlUpdate(){
   let index = location.href.indexOf("lecture");
   chrome.runtime.sendMessage({"message": "inactivate_icon"});
   video = undefined;
   if(index != -1){
      let intervalHandler;
      function findVideo(){
         video = document.getElementsByTagName("video")[0];
         if(video !== undefined){
            clearInterval(intervalHandler);
            chrome.runtime.sendMessage({"message": "activate_icon"});
            video.playbackRate = targetVideoSpeed;
            let videoVar = video;
            video.addEventListener("play", ()=>{
               videoVar.playbackRate = targetVideoSpeed;
            });
         }
      }
      intervalHandler = setInterval(findVideo, 1000);
      findVideo();
   }
}

{
   let lastURL;
   function verityURL(){
      if(location.href != lastURL){
         urlUpdate();
         lastURL = location.href;
      }
   }
   setInterval(verityURL, 1000);
   verityURL();
}

chrome.runtime.onMessage.addListener((message, sender, sendResponse)=>{
   if(message.message == "set playback speed"){
      let speed = message.speed;
      if(speed > 16 || speed < 0){
         console.log(`invalid speed ${speed}`);
         return;
      }
      targetVideoSpeed = speed;
      video.playbackRate = speed;
   }
   else if(message.message == "get playback speed"){
      let intervalHandler = setInterval(()=>{
         if(video === undefined) return;
         chrome.runtime.sendMessage({"message" : "current speed", "speed" : video.playbackRate});
         clearInterval(intervalHandler);
      }, 100);
   }
});