[javascript] YouTube iframe API: how do I control an iframe player that's already in the HTML?

Looks like YouTube has updated their JS API so this is available by default! You can use an existing YouTube iframe's ID...

<iframe id="player" src="http://www.youtube.com/embed/M7lc1UVf-VE?enablejsapi=1&origin=http://example.com" frameborder="0"></iframe>

...in your JS...

var player;
function onYouTubeIframeAPIReady() {
  player = new YT.Player('player', {
    events: {
      'onStateChange': onPlayerStateChange
    }
  });
}

function onPlayerStateChange() {
  //...
}

...and the constructor will use your existing iframe instead of replacing it with a new one. This also means you don't have to specify the videoId to the constructor.

See Loading a video player