-: [Maximilian Schwarzmüller] Now, one other thing we had
in this project was this .d.ts file.
What's that?
Well, that file contains rather strange code,
code that you won't see too often in TypeScript projects
and in the end .d.ts files are TypeScript declaration files
or type declaration files.
And the idea behind such files
is that they don't specify code
that should be executed like main .ts does.
This file here contains a bunch of
JavaScript/TypeScript code
that eventually should be executed after being compiled.
Well, .d.ts files contain just type definitions
that can be used by other files,
but that can't be executed,
so that doesn't compile to JavaScript code,
but that would compile to nothing
because it contains just type definitions, for example.
And this reference thing here,
which is the only code in this .d.ts file,
simply points at some other file
that is part of the vite project.
If you control or command click on vite/client here
you should be taken to this client.d.ts file,
which is part of the vite package,
which is stored in the node modules folder.
And there you see a bunch of type definitions
and this declare thing here,
which in the end just declares TypeScript information.
So that's all just type information,
which can't be executed as JavaScript code.
It's used under the hood to get proper TypeScript support
to avoid errors, to get good IDE support,
but it's not meant to be executed.
You could write .d.ts files on your own
in Project if you knew that you had a file
that only contains type definitions.
But you can also just write a regular .d.ts file
without the .d because you won't get any problems
if you try to compile a file that doesn't lead
to any executable JavaScript code.
We had cases like this before in the course too.
This .d.ts file thing is mostly important for libraries,
so if you're building a library
so that you can ship type definitions
along with your compiled TypeScript code.