D3.js not defined, express serving up static files

I've got express serving up static js files, and a local copy of d3.js (v4) is among them. All the other js files load from my tags, but it says d3 is not defined. I've tried loading from a url too, but no dice, jquery loads fine ("https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js") but not d3 ("https://d3js.org/d3.v4.min.js").

Clearly it is able to locate the resource, because I don't get a 404, but it never runs. When I throw a console.log('hello') into the d3 source, it doesn't show up.

I've tried it in Chrome and Firefox.. I've seen answers that say to use http-server, but isn't express's static server doing the same thing?

Thanks in advance if you've got an idea.

via zbnrg