Example.js

import type {ReducerFn} from '../../../decorators'
import {generateReducer} from '../../../decorators'

class Action {}
class TodoAddMultipleAction extends Action {}
class TodoAddMultipleErrorAction extends Action {}
class TodoAddErrorAction extends Action {}
class TodoCompleteErrorAction extends Action {}
class TodoDestroyErrorAction extends Action {}

class A {}
class B {}

type AnyAction = Object & Action

class Example {
    reduce(a: A, action: AnyAction, b: B): A {
        switch (action.constructor) {
            default:
                return a
        }
    }

    addMultiple(a: A, action: TodoAddMultipleAction, b: B): A {
        return a
    }

    addError(
        a: A,
        {error}: TodoAddMultipleErrorAction
            | TodoAddErrorAction
            | TodoCompleteErrorAction
            | TodoDestroyErrorAction,
        b: B
    ): A {
        return a
    }
}

ExampleOut.js

import type {ReducerFn} from '../../../decorators'
import {generateReducer} from '../../../decorators'

class Action {}
class TodoAddMultipleAction extends Action {}
class TodoAddMultipleErrorAction extends Action {}
class TodoAddErrorAction extends Action {}
class TodoCompleteErrorAction extends Action {}
class TodoDestroyErrorAction extends Action {}

class A {}
class B {}

type AnyAction = Object & Action

class Example {
    reduce(a: A, action: AnyAction, b: B): A {
        
        switch(action.constructor) {
            case TodoAddMultipleAction:
                return this.addMultiple(a, action, b)
            case TodoAddMultipleErrorAction:
            case TodoAddErrorAction:
            case TodoCompleteErrorAction:
            case TodoDestroyErrorAction:
                return this.addError(a, action, b)
            default:
                return a
        }
    }

    addMultiple(a: A, action: TodoAddMultipleAction, b: B): A {
        return a
    }

    addError(
        a: A,
        {error}: TodoAddMultipleErrorAction
            | TodoAddErrorAction
            | TodoCompleteErrorAction
            | TodoDestroyErrorAction,
        b: B
    ): A {
        return a
    }
}