@reduxjs/toolkit/src/mapBuilders.ts

import type { Action, AnyAction } from 'redux'
import type {
  CaseReducer,
  CaseReducers,
  ActionMatcher,
  ActionMatcherDescriptionCollection,
} from './createReducer'

export interface TypedActionCreator<Type extends string> {
  (...args: any[]): Action<Type>
  type: Type
}

/**
 * A builder for an action <-> reducer map.
 *
 * @public
 */
export interface ActionReducerMapBuilder<State> {
  /**
   * Adds a case reducer to handle a single exact action type.
   * @remarks
   * All calls to `builder.addCase` must come before any calls to `builder.addMatcher` or `builder.addDefaultCase`.
   * @param actionCreator - Either a plain action type string, or an action creator generated by [`createAction`](./createAction) that can be used to determine the action type.
   * @param reducer - The actual case reducer function.
   */
  addCase<ActionCreator extends TypedActionCreator<string>>(
    actionCreator: ActionCreator,
    reducer: CaseReducer<State, ReturnType<ActionCreator>>
  ): ActionReducerMapBuilder<State>
  /**
   * Adds a case reducer to handle a single exact action type.
   * @remarks
   * All calls to `builder.addCase` must come before any calls to `builder.addMatcher` or `builder.addDefaultCase`.
   * @param actionCreator - Either a plain action type string, or an action creator generated by [`createAction`](./createAction) that can be used to determine the action type.
   * @param reducer - The actual case reducer function.
   */
  addCase<Type extends string, A extends Action<Type>>(
    type: Type,
    reducer: CaseReducer<State, A>
  ): ActionReducerMapBuilder<State>

  /**
   * Allows you to match your incoming actions against your own filter function instead of only the `action.type` property.
   * @remarks
   * If multiple matcher reducers match, all of them will be executed in the order
   * they were defined in - even if a case reducer already matched.
   * All calls to `builder.addMatcher` must come after any calls to `builder.addCase` and before any calls to `builder.addDefaultCase`.
   * @param matcher - A matcher function. In TypeScript, this should be a [type predicate](https://www.typescriptlang.org/docs/handbook/advanced-types.html#using-type-predicates)
   *   function
   * @param reducer - The actual case reducer function.
   *
   * @example
```ts
import {
  createAction,
  createReducer,
  AsyncThunk,
  AnyAction,
} from "@reduxjs/toolkit";

type GenericAsyncThunk = AsyncThunk<unknown, unknown, any>;

type PendingAction = ReturnType<GenericAsyncThunk["pending"]>;
type RejectedAction = ReturnType<GenericAsyncThunk["rejected"]>;
type FulfilledAction = ReturnType<GenericAsyncThunk["fulfilled"]>;

const initialState: Record<string, string> = {};
const resetAction = createAction("reset-tracked-loading-state");

function isPendingAction(action: AnyAction): action is PendingAction {
  return action.type.endsWith("/pending");
}

const reducer = createReducer(initialState, (builder) => {
  builder
    .addCase(resetAction, () => initialState)
    // matcher can be defined outside as a type predicate function
    .addMatcher(isPendingAction, (state, action) => {
      state[action.meta.requestId] = "pending";
    })
    .addMatcher(
      // matcher can be defined inline as a type predicate function
      (action): action is RejectedAction => action.type.endsWith("/rejected"),
      (state, action) => {
        state[action.meta.requestId] = "rejected";
      }
    )
    // matcher can just return boolean and the matcher can receive a generic argument
    .addMatcher<FulfilledAction>(
      (action) => action.type.endsWith("/fulfilled"),
      (state, action) => {
        state[action.meta.requestId] = "fulfilled";
      }
    );
});
```
   */
  addMatcher<A extends AnyAction>(
    matcher: ActionMatcher<A> | ((action: AnyAction) => boolean),
    reducer: CaseReducer<State, A>
  ): Omit<ActionReducerMapBuilder<State>, 'addCase'>

  /**
   * Adds a "default case" reducer that is executed if no case reducer and no matcher
   * reducer was executed for this action.
   * @param reducer - The fallback "default case" reducer function.
   *
   * @example
```ts
import { createReducer } from '@reduxjs/toolkit'
const initialState = { otherActions: 0 }
const reducer = createReducer(initialState, builder => {
  builder
    // .addCase(...)
    // .addMatcher(...)
    .addDefaultCase((state, action) => {
      state.otherActions++
    })
})
```
   */
  addDefaultCase(reducer: CaseReducer<State, AnyAction>): {}
}

export function executeReducerBuilderCallback<S>(
  builderCallback: (builder: ActionReducerMapBuilder<S>) => void
): [
  CaseReducers<S, any>,
  ActionMatcherDescriptionCollection<S>,
  CaseReducer<S, AnyAction> | undefined
] {
  const actionsMap: CaseReducers<S, any> = {}
  const actionMatchers: ActionMatcherDescriptionCollection<S> = []
  let defaultCaseReducer: CaseReducer<S, AnyAction> | undefined
  const builder = {
    addCase(
      typeOrActionCreator: string | TypedActionCreator<any>,
      reducer: CaseReducer<S>
    ) {
      if (process.env.NODE_ENV !== 'production') {
        /*
         to keep the definition by the user in line with actual behavior, 
         we enforce `addCase` to always be called before calling `addMatcher`
         as matching cases take precedence over matchers
         */
        if (actionMatchers.length > 0) {
          throw new Error(
            '`builder.addCase` should only be called before calling `builder.addMatcher`'
          )
        }
        if (defaultCaseReducer) {
          throw new Error(
            '`builder.addCase` should only be called before calling `builder.addDefaultCase`'
          )
        }
      }
      const type =
        typeof typeOrActionCreator === 'string'
          ? typeOrActionCreator
          : typeOrActionCreator.type
      if (type in actionsMap) {
        throw new Error(
          'addCase cannot be called with two reducers for the same action type'
        )
      }
      actionsMap[type] = reducer
      return builder
    },
    addMatcher<A extends AnyAction>(
      matcher: ActionMatcher<A>,
      reducer: CaseReducer<S, A>
    ) {
      if (process.env.NODE_ENV !== 'production') {
        if (defaultCaseReducer) {
          throw new Error(
            '`builder.addMatcher` should only be called before calling `builder.addDefaultCase`'
          )
        }
      }
      actionMatchers.push({ matcher, reducer })
      return builder
    },
    addDefaultCase(reducer: CaseReducer<S, AnyAction>) {
      if (process.env.NODE_ENV !== 'production') {
        if (defaultCaseReducer) {
          throw new Error('`builder.addDefaultCase` can only be called once')
        }
      }
      defaultCaseReducer = reducer
      return builder
    },
  }
  builderCallback(builder)
  return [actionsMap, actionMatchers, defaultCaseReducer]
}