I spent hours and hours searching for this one, and just by trial and error was I able to finally find the solution. Logging this in Stack Overflow for future searchers.

Q:

How do I create a composite key in indexeddb?
Keys are created in indexeddb on object stores using the following:

var db;
var openRequest = indexedDB.open('myDB', 1);
openRequest.addEventListener('error', () => {
    console.error('Unable to open db');
});
openRequest.addEventListener('upgradeneeded', function(event){
    var store = db.createObjectStore('myStore', 
        {keyPath: /* composite key */ }
    );
});
openRequest.addEventListener('success', function(){
    db = openRequest.result;
});

I have tried placing objects, defining multiple times, how does one create a composite key, or is this a limitation of the API?

Note: If you are looking for how to query a composite key using a range, please check out this post