Mobx State Tree reference type and Typescript

user_781366 0
Viewed 5404

I'm using mobx-state-tree with Typescript in a React application. And, I'm having an issue with Typescript where it complains about the type of the mobx type types.safeReference. It looks like the type of safeReference in the model definition is different from its type when you use .create() to actually create an instance of the model. In my code, selectedProduct's type is converted to string | number | undefined | null in productStore, but in the model definition is IStateTreeNode<...> | undefined | null and that's why I get an error in my root store. How can I fix that?

Here's my Product store:

import { types } from "mobx-state-tree";

const Product = types.model("Product", {
   id: types.identifier,
   name: types.string
})

const ProductStore = types
  .model("ProductStore", {
    products: types.array(Product),
    selectedProduct: types.safeReference(Product),
  })
  .actions((self) => ({
      // actions here
  }));

export const productStore = ProductStore.create({
  products: [],
  selectedProduct: undefined // the type here is different from the type in the actual model
});

And, here's my root store:

import { types } from "mobx-state-tree";
import ProductStore, { productStore } from "./product-store";

const RootStore = types.model('RootStore', {    
  productStore: ProductStore 
})

export const rootStore = RootStore.create({
    productStore: productStore // Here is where I get the typescript error.
});

UPDATE:

Another way of reproducing this issue is by trying to create a custom reference. The getter will complain about undefined not being assignable to type {...}.

const ProductByIdReference = types.maybeNull(
  types.reference(Product, {
      get(id: number, parent: Instance<typeof ProductStore>) {
          return parent.products.find(p => p.id === id) || undefined
      },
      set(value: Instance<typeof Product>) {
          return value.id
      }
  })
)
mobx-state-tree mobx-react mobx typescript reactjs
2 Answers

Generally speaking when you are trying to use a snapshot where an instance is typically used, you use cast. When you are trying to use an instance where a snapshot is typically used, you use castToSnapshot.

export const rootStore = RootStore.create({
    productStore: castToSnapshot(productStore)
});
user_36178860

I checked your problem in detail and I wrote a solution, this is how to use mst rootStore concept correctly, there is no need to create instances of your store or object, just @inject it inside component directly:

// Product Store
import { types, Instance } from "mobx-state-tree";

const Product = types.model("Product", {
  id: types.identifier,
  name: types.string
})

const Store = types
  .model("ProductStore", {
    products: types.array(Product),
    selectedProduct: types.safeReference(Product),
  })
  .actions((self) => ({
    // actions here
  }));

type ProductStoreType = typeof Store;
interface ProductStoreTypeInterface extends ProductStoreType {}
export interface ProductStoreInterface extends Instance<ProductStoreTypeInterface> {}
export const ProductStore: ProductStoreTypeInterface = Store;

// Root Store
import { types } from "mobx-state-tree";
import { ProductStore } from "./product-store";

const RootStore = types.model('RootStore', {
  productStore: types.late(() => types.optional(ProductStore, {})),
})

// And now you can create rootStore with empty object
export const rootStore = RootStore.create({});

// Component
import { inject, observer } from 'mobx-react';
import { SettingStoreInterface } from "../settings/SettingStore";
import React from "react";

interface ComponentProps {}

interface InjectedProps extends ComponentProps {
  productStore: ProductStoreInterface;
}

@inject('productStore')
@observer
class SomeComponent extends React.Component<ComponentProps> {
  componentDidMount() {
    const { productStore } = this.props as InjectedProps;
    
    console.log(productStore.products);
  }
}
user_38160720
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