I'm assuming you're invoking loadMoreOnScroll from within your indexJS.js file, correct?
If so, the solution is to remove the function call from your javascript file and instead call it directly from index.html.
indexJS.js
// Create the function but don't call it here
function loadMoreOnScroll(){...}
index.html
<script src="indexJS.js></script>
<script>
// call the function
loadMoreOnScroll();
</script>
Edit:
A few other people suggested adding a body class and targeting your page that way. This approach is fine, and may work well in many scenarios but just keep in mind two things:
This works well for if you need to call your function on only one or two pages. Any more and you'll have to maintain a growing list of body classes within indexJS.js.
Using the body class as a hook decouples the function call from the page that its applies to.
In other words, the body class will have functionality tied to it that's not immediately obvious if you're only looking at the HTML. If you're working on the code yourself, you'll probably be ok, but in a team environment, it could be error-prone. (Or if you revisit the code after a few months). It all depends on the scope of your project.