Import Socket.IO Client in the Content Script of a Chrome Extension

Yes, there is a similar question to this already has already been answered (Chrome Extension Socket io node js); however, it only works for background scripts (from what I've seen). So, how would I do this in the content script?

Here is what I have so far:

// manifest.json
// And yes, I've also tried to import "socket.io.js" as a background script

"content_scripts":
[
  {
    "matches": ["<all_urls>"],
    "js": ["socket.io.js", "index.js"]
  }
]

// index.js

const socket = io('http://localhost:8080');

socket.on('some-event', (data) => {
  ...
};

//  index.js:1 Uncaught ReferenceError: io is not defined
//    at index.js:1

What am I doing wrong?

Thanks.