Hi jbeat497 
The question you’re asking is one more of learning the basics of JavaScript and how to fetch
than one specifically oriented at how to use Netlify or its tools. That’s why @perry has to say
We can’t, as a general rule, help you learn, write, or debug code (there are sadly only 8 support engineers for hundreds of thousands of sites worldwide)
So as a general point of direction, I would recommend picking up a course online (there are tons of free ones available!) for learning JavaScript! It’s a fun journey. In the meantime, here’s a quick answer. Please don’t take this and expect to run - I fully recommend doing a proper foundational course on the language.
fetch('/.netlify/functions/mostpopular', {
method: 'POST',
body: JSON.stringify({
something: [
"foo",
"bar",
"baz"
]
})
})
fetch
takes a second argument of options for request data, which should contain a body
for the actual body of the request, which ought to be stringified JSON. An array is a valid json value provided it has a valid key.
Give this a thorough read: https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API