How do I know which version of Javascript I'm using?

Click on this link to see which version your BROWSER is using: http://jsfiddle.net/Ac6CT/

You should be able filter by using script tags to each JS version.

<script type="text/javascript">
  var jsver = 1.0;
</script>
<script language="Javascript1.1">
  jsver = 1.1;
</script>
<script language="Javascript1.2">
  jsver = 1.2;
</script>
<script language="Javascript1.3">
  jsver = 1.3;
</script>
<script language="Javascript1.4">
  jsver = 1.4;
</script>
<script language="Javascript1.5">
  jsver = 1.5;
</script>
<script language="Javascript1.6">
  jsver = 1.6;
</script>
<script language="Javascript1.7">
  jsver = 1.7;
</script>
<script language="Javascript1.8">
  jsver = 1.8;
</script>
<script language="Javascript1.9">
  jsver = 1.9;
</script>

<script type="text/javascript">
  alert(jsver);
</script>

My Chrome reports 1.7

Blatantly stolen from: http://javascript.about.com/library/bljver.htm

Wikipedia (or rather, the community on Wikipedia) keeps a pretty good up-to-date list here.

• Most browsers are on 1.5 (though they have features of later versions)
• Mozilla progresses with every dot release (they maintain the standard so that's not surprising)
• Firefox 4 is on JavaScript 1.8.5
• The other big off-the-beaten-path one is IE9 - it implements ECMAScript 5, but doesn't implement all the features of JavaScript 1.8.5 (not sure what they're calling this version of JScript, engine codenamed Chakra, yet).

In chrome you can find easily not only your JS version but also a flash version. All you need is to type chrome://version/ in a command line and you will get something like this: