Hey@evelant - thanks for putting together this issue. I'm sorry it's taken so long for us to get back to you.

TypeScript inference is a known problem with MST, and it's definitely something we're trying to improve. That said, I don't have a great fix for you right now off the top of my head.

If you found a solution to this, it would help a lot for others to see it. If you didn't and moved past it, no worries at all.

For now, I'm going to mark this as a TypeScript issue, label it that help is welcome, and hopefully we can find a path forward to make this kind of thing easier.

More info/perhaps related - TypeScript seems to have trouble inferring things about nested models intypes.map overall.

In this CodeSandbox, TypeScript complains about thetodoStore types, even though they should be satisfactory.

But in this CodeSandbox, TS is happy with us fortypes.array

An example a little closer to the original problem:https://codesandbox.io/s/clever-margulis-ks8fx7?file=/src/App.tsx.types.array is working here.

Ooh, butarray.filter method is typed withany, so maybe if we write a customfilter method this would break, too. I have to hop off for a while but I'd be curious if anyone else looks into that to see if this is just an issue with the generics overall.

I will check tomorrow

I have looked into the map implementation, there are few places withany tried fixing them, but still couldn't find the root where the types are getting changed

Thanks@chakrihacker! We can keep this open and maybe revisit it over time. Lots of TypeScript work to be done, haha!

Thanks@thegedge - this is an excellent solution. Really appreciate your insight.

I'm going to convert this issue to a discussion, which will close it. I forked the original reproduction and made theIterable change, and thing are working over here:https://codesandbox.io/p/sandbox/inspiring-neco-5t5dsq

Here's the code for posterity:

import"./styles.css";import{typesast}from"mobx-state-tree";/** * Like array.filter except on a map, returns value[] array of found results *@param m *@param predicate *@returns */exportfunctionfilterMapValues<K,V>(m:Iterable<[K,V]>,predicate:(v:V)=>any):V[]{constres:V[]=[];for(const[k,v]ofm){if(!!predicate(v)){res.push(v);}}returnres;}constfirstModel=t.model({}).views((self)=>({getfoo():string{return"foo";},}));constsecondModel=t.model({myMap:t.map(firstModel),});constm=secondModel.create({myMap:{test:{}}});//Views don't exist when extracting the value type of//an instance from a map in a generic function.//Instead we seem to get only the props.constf=filterMapValues(m.myMap,(val)=>val.foo==="foo");exportdefaultfunctionApp(){return<divclassName="App"></div>;}