Update:

Now that I use prettier, it's really a matter of what you like to look at (because prettier means that I don't have to type them at all). I prefer the way code looks without semicolons so… :) I just use eslint-config-prettier and let prettier deal with it.

Semicolons in JavaScript has got to be one of the worst bikeshedded topics of all time (right after spaces vs. tabs... 2 spaces please). Here are three resources on the subject of why semicolons are not necessary. Here's some criteria that need to be in place before I will sanction omitting semicolons in a project.

Compilation and/or Uglification

The first thing that you need to know is something called Automatic Semicolon Insertion (ASI). It's the "feature" that allows us to even have this conversation. Read up on that if you're not familiar. You should not rely on ASI. It's a really bad idea for many reasons.

The problems with relying on ASI go away when you compile or minify your code (depending on your technology). For example, compiling with Babel will add the semicolons back and uglifying with terser will too.

So for me to say you're good to go on omitting semicolons in your source code, you first need to make sure that whatever you end up shipping to production (whether browser or node) has the semicolons added back.

Linting the bad parts

There are a few gotchas with ASI. However, if you are using ESLint (which you should) and you enable the no-unexpected-multiline rule, then you're safe. Just make sure that your build pipeline will fail if that rule is broken because most assuredly your app will!

Why omit semicolons anyway?

With these things in place, this is no longer a discussion about what works and what doesn't but becomes a simple matter of preference.

Omitting semicolons is a matter of preference

So why do I prefer to not have semicolons? It's not just that I have a broken right pinky (though sometimes it gets hurt) or I like typing one less character per line. It's simply because I don't like my linter/editor telling me I need to add something that is not necessary.

Also, I like to stay focused on the problem, not worrying about adding or removing something that doesn't really matter in the end. And since I've started omitting semicolons (and gotten used to how ugly it looks at first) I actually feel like it leaves my code looking cleaner (you've just gotta be untrained to think that you need semicolons).

Why should you use semicolons?

One argument I've heard for using semicolons is that without them, it's harder for newcomers to understand the code. I suppose it's possible, but that hasn't been my experience with newcomers I've taught. For me I don't feel like my code has gotten any less clear, maintainable, or readable since I removed semicolons from it. And now I don't have to even think about it.

Conclusion

If you don't compile/uglify and lint your code properly then I do not recommend you omit semicolons in your code (it's not a matter of preference in this case, it's simply the proper way to write JavaScript). I definitely would recommend that you get these things in place (but that's another blogpost). If you do have these things in place, then that's great! You can make the choice based on preference! Catch you on the twitters!