The only downside seems that if one uses new keywords like "class" one has to use a transpiler back to JS5 to avoid syntax errors in IE11 and other older still supported browsers&devices that will never receive an update (like Android 2x/4, Blackberry, WinPhone7/8, etc).
A fallback solution like JQuery isn't possible for new keywords. Try-catch keywords were previously introduced too, but are rarely used because their implementation is known to be slow. Beside that JS5 code with fallback functions still works fine in very old browsers like IE5.
A fallback solution like JQuery isn't possible for new keywords. Try-catch keywords were previously introduced too, but are rarely used because their implementation is known to be slow. Beside that JS5 code with fallback functions still works fine in very old browsers like IE5.