If you are one of those who doesn’t have time to test, then the least you can do is to write major integration tests and work your way down to smaller chunks, unit tests.
Example would be to make a post request to your ajax call and expect the proper return. That’s the simplest thing you can do to get some comfort when deploying to production.
This will at least protect you from 500 errors.
Then when you have more time, break those down to unit tests