Ask HN: How to scrape your own cookies,cache a la product recommendation APIs?

I would like to build a product focused on scraping data that average internet users generate in their normal internet footprint. Basically, I want to know what is the best practices and the engineering required to scrape and process data from caches, internet cookies, and such. If I could know what area of development this is, and some libraries to get started(esp. in JS) I would greatly appreciate it.

Thank you!