Aside from newer methods the only syntax gotchas in my code are template strings and let/const. That makes the code really quick and simple to convert to an old IE friendly application.

alert("Reached here #23");

More like:

//alert('DEBUG: SomeFunction: condition is TRUE');

Then, depending on what browser I'm debugging in, the post-processor changes that to either console.log, document.title=, or alert(

:)