Transcript from the "Declaration Files & Type-only Imports" Lesson
>> So the last category of things, and there's some details I'm skipping over here, some things that I feel are potentially less relevant to the everyday TypeScript user. But the last category of things that have been added to TypeScript since the last course was recorded are really two things.
[00:00:52] So now you could conceivably have set your code base up that way. And if it were a library, as of TypeScript 3.7, you can generate declaration files, those DTS files. So consumers of your library will be able to get all the benefits as if you wrote your library in TypeScript.
[00:01:47] There are just fewer and fewer things where, you don't get that benefit unless you're using a TS file. Finally, last thing I wanna mention is the concept of type-only imports. I will show you how this works when we get into the code base, but it's really just something like the following.
[00:02:05] import type, and then we find the file. So we do it like this. I've got a module here, and I can just say, I simply want the type information from this module, I'm not importing the value. So if I were here to try to invoke this, I think I should get an error.
[00:02:43] There we go, it cannot be used as a value because it was imported using import type. Why is this important? Well, raise your hand if you use something like Parcel or Webpack as your module bundler in your app. import type allows us to consume type information only. We took a show of hands in the class, and turns out a lot of people use Webpack or Parcel, things like this.
[00:03:11] So these module bundlers, they rely on basically analyzing the code that you import as a mechanism of splitting things up. So you don't need to send more code to your users, especially for browser-based apps. Don't wanna send more code to them than they need for maybe that first page load.
[00:03:29] Import type is a way that we can refer to type information in another module, without saying, I actually need that value. I need that function because I plan to invoke that function, right? So this won't sort of trigger additional code being included in your bundle. Type information's just a build-time construct, so this is a nice way of making sure that everything remains loosely coupled.