My understanding is that it uses the new Salsa JavaScript service.
This blog link (provided by i100) contains the following statement in the Known Changes section:
Another key difference is that while the old language service supports an extensibility model that allows you to customize extensions for specific third-party JavaScript libraries, Salsa no longer supports these extensions in favor of TypeScript definition files, which encourage collaborative collaboration and are easier to maintain relevance.
Although the wording is somewhat ambiguous for me, I assume that this means adding javaScript libraries as links is not directly supported in VS 2017. The only way to get Intellisense in VS 2017 is with a TypeScript definition file.
I found this question on how to generate TypeScript def files from existing JavaScript files. The usual wisdom is that TS def files require more specificity than JavaScript files, and therefore they are most reliably created manually. However, this may not be an acceptable solution if you are like me and have tens of thousands of lines of custom library definitions.
There are several programs that are said to automatically generate def files for TS files, but I have never used them and cannot talk about their effectiveness.
dts-gen is created by Microsoft.
The dts-generator is created by SitePen.
It seems that the industry believes TypeScript is the way of the future, and are working hard to make TS standard development standard.