Does Nestjs use some sort of its own compiler?

[fringe token]

I'm just starting out with learning nestjs and playing around with examples from the docs

This snippet of code works, cats module defines providers and controllers etc.

@Controller('cats')
export class CatsController {
  constructor(private readonly catsService: CatsService) {}
// etc...

BUT
If I remove the type annotation

@Controller('cats')
export class CatsController {
  constructor(private readonly catsService) {}
// etc...

The app breaks - since I'm used to type annotations not affecting runtime at all I have to wonder why does it matter. My only guess would be that Nestjs implements its own compiler? or I've heard typescript allows to write plugins to the compiler - something like that?

this is just a question out of curiosity how nestjs works

[rocky forge]

@fringe token - Do you know how TypeScript works with JavaScript?

[fringe token]

that's a very broad question - I mean I'm aware that ts compiles to js. It does some type-checking, strips away the types, converts some ts specific features(like decorators or property modifiers in constructors) to js compatible code, maybe a little bit of transpilation - something like that.

Never have I encountered a scenario where a type annotation would affect the resulting code - except for a compilation error due to type-checking.
The closest thing I saw was a library called Typia which used ts compiler plugins to use type annotations to construct runtime validators.

My guess was that also maybe since class becomes unused the build tools strip away the import and that breaks somehow but this snippet also crashes

@Controller('cats')
export class CatsController {
  private readonly catsService: CatsService;
  constructor(catsService) {
    this.catsService = catsService;
  }
// etc...

[rocky forge]

Ok. Just needed to understand your level of knowledge. The piece you are missing is how Nest uses reflect-metadata to use TypeScript for its initialization/ bootstrapping process. I'm not 100% in the know about how the internals works, as it is Nest's black-box feature, but Nest needs the types to basically know what to wire up. That's my understanding. @hushed heron Jay could probably explain better and correct me if I am wrong. 🙂

This article might help get you a better understanding: https://blog.bitsrc.io/typescripts-reflect-metadata-what-it-is-and-how-to-use-it-fb7b19cfc7e2

[fringe token]

Yeah, I saw that you use reflect in decorators to set-up controllers and stuff - didn't know it gets access to type annotation data tho, I thought it was limited to prototype, property name - that kind of thing

Ok, I see - "design:paramtypes" metadata in Reflect has info on type annotation it seems, thanks

[ripe herald]

This post has been marked as resolved. :white_check_mark:
Please read through the conversation and resolution if you are having the same issue, and then re-open the post if you are still having trouble, providing as much extra information as possible.