title: Which Edition of ECMA-262 Does Google Apps Script Support?
body: <p>According to <a href="http://productforums.google.com/forum/#!searchin/apps-script/ecma-262/apps-script/poDhzsvZWHU/iKmrgsNCOToJ">this thread</a> from the old Google Group, Apps Script is based on ECMA-262 3rd Edition. </p>
  
  <p>This would seem to be supported by the fact that auto-completion in the editor displays 3rd Edition array functions.</p>
  
  <p>However the following code runs perfectly well, which casts doubt on the matter:</p>
  
  <pre><code>var array = [
    1,2,3,4,5
  ];
  
  Logger.log("forEach:");
  array.forEach(function (item,idx,arr) {
    Logger.log(item); 
  });
  </code></pre>
  
  <p>Note the use of ECMA-262 5th Edition Array function forEach.</p>
  
  <p>Would someone authoritative mind giving a definitive answer on why this is the case? And whether it's safe to rely on all 5th Edition features or a sub-set that have been implemented and seem to work?</p>