That means wrapping your code with a function called like we have here. Now that's because we often run processors that will combine multiple pieces of scripts that we have in our applications. Without an IIFE, when this is combined with some other script, that script will also run in strict mode even if it wasn't meant to, if you don't add these function wraparounds. If you need more information about how IIFE's work, check out an earlier episode on how to work with them.
Now of course, strict mode can also run inside individual functions. And then, only those functions will run in strict mode, so you can use it in a more targeted way. This is also going to make any functions that are nested inside this one also run in strict mode as well. The simplest example is to show the issue with variables needing to be declared, but just in case you're asked to provide some other examples in your interview, strict mode prevents you from making some assignments which sloppy mode would, shockingly enough, not mind at all.
Now there's a few other ones, but those are the main ones that you should worry about. You can find most of the issues at the Mozilla Developer Network strict mode page. Here's a couple of links where you can get more information about working with strict mode, as well as links to previous episodes of the shows that I mentioned earlier. If you have some ideas for this weekly series, maybe you want to share with me some questions you've been asked, or have asked in interviews, connect with me in LinkedIn, or just about any other social media network, like Twitter or GitHub, at Planet of the Web.
Skill Level Intermediate
Q: Why can't I earn a Certificate of Completion for this course?
A: We publish a new tutorial or tutorials for this course on a regular basis. We are unable to offer a Certificate of Completion because it is an ever-evolving course that is not designed to be completed. Check back often for new movies.