Declarative class-based reducers for redux and NGRX

So, as a Redux maintainer...

I completely do _not_ see the point of trying to write reducers as classes.

A reducer is a pure function. There shouldn't be any use of `this` inside. Therefore, it doesn't need to be defined as a class instance method. Some kind of a factory function, as recommended in the docs? Sure.

Let me specifically address the points of concern about the `createReducer`-type approach:

>- In case of complex reducers we have to leave lots of comments describing what this reducer does and why.

I don't see how this changes no matter where or how you define the reducer.

> - Huge reducer maps are hard to read.

If this is truly a concern, then try defining the functions outside the lookup table.

> - Each reducer has only one corresponding action type. What if I want to run the same reducer for several actions?

Then define the functions outside the lookup table, and associate them with multiple keys.

Going a step further beyond this, our new Redux Starter Kit package [1] has a `createSlice` function [2] that will auto-generate action types and action creators for reducer functions that are passed in, and _also_ allows you to pass in reducers to handle action types that were defined elsewhere. Example:

    const counter = createSlice({
      slice: 'counter', // slice is optional, and could be blank ''
      initialState: 0,
      reducers: {
        increment: state => state + 1,
        decrement: state => state - 1,
        multiply: (state, action) => state * action.payload
      }
    })

    const user = createSlice({
      slice: 'user',
      initialState: { name: '', age: 20 },
      reducers: {
        setUserName: (state, action) => {
          state.name = action.payload // "mutate" the state all you want with immer
        }
      },
      extraReducers: {
        [counter.actions.increment]: (state, action) => {
          state.age += 1
        }
      }
    })

[0] https://www.reddit.com/r/javascript/comments/awk2be/reducer_...

[1] https://redux-starter-kit.js.org/

[2] https://redux-starter-kit.js.org/api/createSlice