Initial support of traits in Typescript

[bjarkehs]

In attempt to fix #524 this adds a very simple form of support for traits in Typescript.

Currently missing any handling of trait properties.

Downside of this is that it currently does not handle the traits' added properties.

I attempted to type the trait function and FlattenFactoryMethods so I could filter out the trait and instead add the trait's properties. But I couldn't figure it out.

I attempted something like:

// Extract factory method return values from a factory definition
type FlattenFactoryMethods<T> = {
  [K in keyof T]: T[K] extends (n: number) => infer V
    ? V
    : T[K] extends (data: {}) => Trait<infer U>
    ? U
    : T[K];
};

type Trait<T extends {} = {}> = T & { __isTrait__: true };

This is not part of the PR since I couldn't get it working.

[samselikoff]

@dfreeman @chriskrycho @zoltan-nz @jamescdavis If any of you have any spare cycles to take a look at this I'd appreciate it!

If not feel free to ignore :)

[samselikoff]

@bjarkehs thanks for opening this!

[dfreeman]

I should have some time to look at this later this week—first I'll have to read up on how traits actually work 😅

[samselikoff]

Oh man traits are the bomb. Similar to FactoryBot traits if you've ever used em.

https://miragejs.com/docs/main-concepts/factories/#traits

[jamescdavis]

I'm hoping to look at this week as well. And I'm pretty familiar with traits. 😉

[dfreeman]

Just a quick update on this: I spent some time over the weekend puzzling out how to type the way traits work. It seems like it's possible, but... complicated 😄

I'll coordinate with @jamescdavis and see if we can't get a cleaned up version put together to propose here

[samselikoff]

Cool – feel free to propose new API if it helps too (like we did with createServer)

[thedaviddias]

@jamescdavis @dfreeman any update in adding trait and association?

[yekver]

really waiting for this PR!

[MrLoh]

so am I any progress on maybe at least not throwing a type error for this anymore

[rubenmoya]

Anything we can do to push this forward? Even without having types in the traits properties at least it won't throw an error when trying to use traits.

[dfreeman]

I picked this up to work on it over the holidays and wound up a little stumped by the change that landed in #695 and what problem it was addressing. Had a quick conversation in the #typescript channel about it and Sam suggested checking with the author of the PR.

@NLincoln can you offer any insight into that fix? I see you linked to some example code that doesn't work because factories don't understand ES getters, but I don't quite understand how the types change addresses that. I'm also happy to chat on Discord if that would be easier than going back and forth on issue comments 🙂

[josemarluedke]

Looking forward to having TypeScript support for traits!

[josemarluedke]

Is there any way to also add type for afterCreate method?

[wagenet]

@dfreeman it definitely seems incorrect to me as well. See #695 (comment)

[jasikpark]

Wouldn't

/** Declares traits for Factory */
  export function trait<Data>(data: Data): Data & { __isTrait__: true };

be slightly more correct, since trait is just adding that value to Data?

or

type Trait<T extends {} = {}> = T & { __isTrait__: true };
/** Declares traits for Factory */
  export function trait<Data>(data: Data): Trait<Data>;

a la @bjarkehs's original snippet in the PR description