speed / mirroring tools for Youtube

(useful for learning dance from videos, lecture videos, etc.)

note

Kind of broken as of 18 jan 2026 because of various updates to youtube. last update at least preserves speed up/slow down and start/end time functionality.

to activate

Set the following as a bookmark in any browser and 'click' the bookmark on a Youtube video.

javascript:(function()%7Bdocument.body.appendChild(document.createElement(%27script%27)).src%3D%27https://cdn.rawgit.com/lzil/yt-dance/master/yt-dance.js%27%3B%7D)()%3B

NOTE: might not work anymore because of CSP issues. In that case need to minimize the JS and put it in the bookmark:

javascript:(function(){var m=false,n=null;function u(e){var t=parseInt(e/60),r=parseInt(e-60*t),i=parseInt(100*(e-60*t-r));return r<10&&(r="0"+r),i<10&&(i="0"+i),t+":"+r+":"+i}function f(){m?document.body.removeChild(n):(n=document.createElement("style"),document.body.appendChild(n),n.innerHTML=".xflip{-moz-transform:scale(-1,1)!important;-webkit-transform:scale(-1,1)!important;-o-transform:scale(-1,1)!important;transform:scale(-1,1)!important;filter:FlipH!important;}"),m=!m}function main(){if(!on_lock)return;var b=document.getElementsByClassName("ytp-chrome-top-buttons")[0],s=document.createElement("div");s.textContent="[1x]",s.style.cssText="background-color:rgba(0,0,0,0.3);font-size:25px;left:0;position:absolute;padding:0 5px;top:10px",b.appendChild(s);var t=document.createElement("div");t.style.cssText="background-color:rgba(0,0,0,0.3);font-size:15px;left:0;position:absolute;padding:0 5px;top:42px",b.appendChild(t);var a=document.createElement("a");a.href="https://github.com/lzil/yt-dance",a.target="_blank",a.textContent="press for help";var i=document.createElement("div");i.style.cssText="background-color:rgba(0,0,0,0.3);font-size:15px;right:0;position:absolute;padding:0 5px;top:10px",i.appendChild(a),b.appendChild(i);var v=document.querySelector("video"),p=document.getElementById("movie_player"),ts=0,ts_t="start",te=null,te_t="end",r=false,info_toggle=true;v.ontimeupdate=function(){var c=p.getCurrentTime(),e=null==te&&c>=v.duration||null!=te&&c>te&&c<te+v.playbackRate;!v.paused&&e&&(r?p.seekTo(ts):v.pause())};window.onkeydown=function(e){if("INPUT"!==e.target.nodeName){var c=v.playbackRate,time,ok=false;switch(e.keyCode){case 83:ok=true,ts!==0?(ts=0,ts_t="start"):(time=p.getCurrentTime(),(null==te&&time>=v.duration||null!=te&&time>=te)||(ts=time,ts_t=u(time)));break;case 69:ok=true,null!==te?(te=null,te_t="end"):(time=p.getCurrentTime(),time>ts&&(te=time,te_t=u(time)));break;case 71:ok=true,p.seekTo(ts,true);break;case 72:ok=true,info_toggle=!info_toggle,info_toggle?i.appendChild(a):i.removeChild(a);break;case 68:ok=true,f();break;case 80:ok=true,c+=.1;break;case 81:ok=true,c-=.1;break;case 82:ok=true,c=1,m&&f(),ts=0,te=null,r=false;break;case 90:ok=true,on_lock=void 0,b.removeChild(t),b.removeChild(s),b.removeChild(i),v.playbackRate=1,m&&f(),v.ontimeupdate=null,window.onkeydown=null;break;case 186:case 59:ok=true,r=!r}if(ok){v.playbackRate=c,s.textContent=" ["+Math.round(10*c)/10+"x"+(m?", mirrored":"")+"]";var l=ts_t+" - "+te_t;r&&(l+=", replay on"),t.textContent=l==="start - end"?"":l}}}}try{on_lock||(on_lock=true,main())}catch(e){on_lock=true,main()}})();

to use

• p to speed up by 0.1x
• q to slow down by 0.1x
• d to mirror (press again to unmirror)
  
  
• s to save current timestamp as START_TIME (default: start of video)
• e to save current timestamp as END_TIME (default: end of video)
• press s/e again to reset START_TIME and END_TIME to defaults
• g to go to START_TIME
• ; toggles auto-replay from START_TIME once video reaches END_TIME
  
  
• . to move forward one frame (base YT functionality)
• , to move backward one frame (base YT functionality)
  
  
• r to reset to 1x unmirrored (original video), no replay, start or end times
• h to toggle help link
  
  
• z to remove this javascript from the video

troubleshooting

• it's potentially interacting with another plugin you have. for instance if used with Chrome's youtube speed plugin then things will potentially break
• for other issues, or comments, contact me at liangz [at] gatsby.ucl.ac.uk