Looks good, thanks. Quick question before merging: was this working before (or is it working now)? If so, does that mean that Worker’s Set implementation just happened to support a .length alias as well?

Try this in your JS console:

var mySet = new Set();
mySet.add(1);
mySet.add(5);
mySet.add('some text');

console.info(mySet.length); // undefined

console.info(mySet.size); // 3

You'll see that length fails. I've updated the commit so that the JSON output format remains the same (so more of a "refactor" than a "fix").