Extremely a lot!!!
Specify some rules and stick to them as a team. You should establish a development environment by using some standards (for example in js development):
These will make code reviews faster and the whole codebase will become much more maintainable. It will look like as if the same guy (you) wrote all of this code. That way everyone will feel extremely familiar and comfortable with it. Refactoring cycles will last less and your team's level will improve much faster.
Also consider that a messy codebase will make you frustrated every morning you open your computer. This is counter-productive and you 'll lose much time fixing ugly things here and there in random. If you think some more about it, you are wasting your time because you may beautify a file today and a teammate will probably break it again by overriding your fixes. This includes simple things like indentation and quite complicated like ES7+ features implementation. Common rules are really important so you avoid doing circles over and over again!!
As a developer many times your bosses will push you to make things faster and faster. Unfortunately, this is true since many product / project managers don't care about the codebase itself or they don't have any programming experience so they are focused only on revenue etc If you are forced to take shortcuts again and again by lowering your standards, find a better working environment and walk away. Sooner or later this wrong mindset will harm you and this project a lot.