Link index.html client.js and server.js

The browser (because you have <script src="/client.js">) makes a request for /client.js

The server:

1. Gets the request
2. Runs response
3. Reads index.html
4. Sends it to the browser

Since index.html starts with <, the browser throws an error when it tries to run it as JavaScript.

Why are you giving the browser index.html when it asks for client.js?

You need to examine the request object, determine what URL is being asked for, write logic to return the correct resource with the correct status code and the correct content-type, and then return that to the client.

The Node.js documentation has an example of this but you should probably stop trying to use createServer directly — since it involves a massive amount of wheel reinvention — switch to using Express and work through the (very short) getting started guide which includes a section on using the static module to serve up static files.