D3 Choropleth Map challenge

Can someone explain how the datasets are matched together.
You have a list of counties and a list of zip codes?? (“fips”) whatever that means.

Peaking at the solution I can see this is how it was matched but I don’t understand it

let result = education.filter((obj) => obj.fips == d.id);

The counties dataset doesn’t have an id property as far as I can tell so result is always null.

My attempt is here

FIPS are the Federal Information Processing Standard Publication codes.