Error 6412 - Cannot assign to read only property

I am trying to update the state model, but I am running into an issue (because I do not have enough experience with it yet). Getting "core.mjs:6412 ERROR TypeError: Cannot assign to read only property 'decisionDate' of object '[object Object]'".

The issue is at the line I added "-->" to. What have I done wrong/what am I missing?

@Action(ClerkAction.Review.UpdateSelectedDate)  
    onUpdateSelectedDate(ctx: StateContext<ClerkStateModel>, action: ClerkAction.Review.UpdateSelectedDate) {
        const DocumentList = ctx.getState().request.documents;
        const DocumentIndex = DocumentList.findIndex(item => item.guid === action.documentGuid);  
-->     DocumentList[DocumentIndex].decisionDate = action.newDate;  

		ctx.patchState({ 
			request: {
				...ctx.getState().request,
				documents: DocumentList
			}        
        });
	
		ctx.dispatch(new NotificationAction.Loading(false));
    }  

UPDATE and SOLUTION - 07-03-2023
So, I had to:

• I had to clone the original item (use the react ...)

• Then I had to replace the existing element of the collection, with the newly cloned element (along with its change).

• Then patchState

    const DocumentList = ctx.getState().request.documents;
    const DocumentIndex = DocumentList.findIndex(item => item.guid === action.documentGuid);
    var ItemToUpdate = {...DocumentList[DocumentIndex]};
  
    ItemToUpdate.decisionDate = action.newDate;
  
    const reconstruct = addOrReplace('guid', DocumentList, ItemToUpdate);
  

Hope this helps someone.

Another option is to use StateOperators

In your case it should be something like:

> ctx.setState(
> patch<ClerkStateModel>({
> documents: updateItem(
> item => item.guid === action.documentGuid,
> patch({ decisionDate: action.newDate }
> )
> })
> );