I'm using some client-side JavaScript to make some API calls that return data to an end user. I'd like to use a Webhook (rather than polling the API) to get updated information from the API when it's available, but not sure if this is even since my JS is only client-side and has to be lightweight (e.g. I rather not rely on loading any libraries).
Would appreciate any pointers.
There is nothing wrong with loading libraries. Todays webpages heavily rely on libraries and albeit you can overdo it, loading a library is nothing which makes your page "heavy". You are better off doublechecking your image sizes when you optimize for size.
For webhook implementations I would recommend websockets. A nice libarary for this purpose is socket.io. With websockets you can push data from the server to the client so you dont rely on polling.