Immutable drafts with immer

Describe the bug

Hi! I am trying to use simpler-state with immer and TypeScript in React Native and I am not able to mutate the document draft. Here’s the relevant code:

interface State {
  readonly plants: [
    {
      readonly name?: string,
    }?
  ]
};

const initialState: State = {};

export const state = entity(initialState);

export function addPlant() {
  state.set(produce(draft => {
    draft.plants = draft.plants || [];
    draft.plants.push({});
  }));
}

This code results in the following error:

Unhandled promise rejection: TypeError: Attempted to assign to readonly property.]
at node_modules/immer/dist/immer.esm.js:1:15732 in <anonymous>
at node_modules/immer/dist/immer.esm.js:1:16123 in f.then$argument_1
at node_modules/immer/dist/immer.esm.js:1:15694 in <anonymous>
at node_modules/simpler-state/lib/entity.js:40:49 in createSetter
at node_modules/simpler-state/lib/persistence.js:26:16 in <anonymous>
at node_modules/react-native/Libraries/Pressability/Pressability.js:691:17 in _performTransitionSideEffects
at node_modules/react-native/Libraries/Pressability/Pressability.js:628:6 in _receiveSignal
at node_modules/react-native/Libraries/Pressability/Pressability.js:524:8 in responderEventHandlers.onResponderRelease
at [native code]:null in forEach
at [native code]:null in callFunctionReturnFlushedQueue

I am trying to initialize the plants array if it doesn’t already exist.

To Reproduce

Run addPlant above.

Expected behavior

I would expect the whole draft document to be mutable.

[ottobonn]

Hi again! I just figured it out. I was using the persistence option with SimpleR State to save the JSON to the filesystem, but I didn’t notice that it handles stringification for me. As a result, I was double-serializing the JSON saved to the filesystem, and when I loaded it I wasn’t deserializing it, so it was a string instead of an object. When I tried to mutate it with immer, I was actually trying to mutate a string, so it was throwing an error.

Thanks for the reply and for the great library!

[arnelenero]

Thanks for posting the details. This could help others who, in the future, might also run into a similar scenario.