how to link html file and index.js code example

Example 1: link js file to index.html

<script src="your js file"></script>

Example 2: How to add js file to a site through url

var jsFile = document.createElement("script");
jsFile.src = "homePage.js";  // it can be path also  "{themes('/scripts/homePage.js')}"
document.body.appendChild(jsFile);  //append it as src to body