@johngw/stream/dist/es/transformers/stateReducer.js

Reactive programming tools using the WHATWG Streams API.

import { StateReducerInit, } from '#transformers/stateReducer/Reducers';
/**
 * Consumes actions and queues changes to a piece state with provided reducers.
 *
 * It's important to note that if the result of an action does not change
 * the state then nothing is queued to the stream.
 *
 * @group Transformers
 * @see {@link StatefulSubject:class}
 * @example
 * ```
 * ---------------{'add author','Jane Austin'}--|
 *
 * stateReducer({
 *   __INIT__: () => ({ authors: [] }),
 *   'add author': (state, author) => ({
 *     ...state,
 *     authors: [...state.authors, author]
 *   })
 * })
 *
 * --{authors:[]}--{authors:['Jane Austin']}-----
 * ```
 *
 * @example
 * ```
 * // The State will be the type of data you want to manipulate.
 * interface State {
 *   authors: string[]
 * }
 *
 * // The Actions are a record of "action name" to a parameter.
 * // If the action doesn't require a parameter, use `void`.
 * type Actions = {
 *   'add author': string
 *   'action that doesnt need a parameter': void
 * }
 *
 * const controllable = new ControllableStream<StateReducerInput<Actions>>()
 *
 * // To get the best out of this API, pass the Actions type and State
 * // during invocation.
 * controllable
 *   .pipeThrough(stateReducer<Actions, State>(
 *     {
 *       __INIT__: () => ({ authors: [] }),
 *
 *       'add author': (state, author) => state.authors.includes(author) ? state : {
 *          ...state,
 *          authors: [...state.authors, author],
 *       },
 *
 *       'action that doesnt need a parameter': (state) => state
 *     }
 *   ))
 *   .pipeTo(write(console.info))
 * // { action: '__INIT__', state: { authors: [] } }
 *
 * controllable.enqueue({ action: 'add author', param: 'Jane Austin' })
 * // { action: 'add author', param: 'Jane Austin', state: { authors: ['Jane Austin'] } }
 *
 * controllable.enqueue({ action: 'add author', param: 'Jane Austin' })
 * // **Nothing will be queued as the state did not change.**
 * ```
 */
export function stateReducer(reducers) {
    let state = Object.freeze(reducers[StateReducerInit]());
    return new TransformStream({
        start(controller) {
            controller.enqueue({
                action: StateReducerInit,
                state,
            });
        },
        transform(chunk, controller) {
            let $state;
            try {
                // No idea why `chunk` (`StateReducerInput`) doesn't have typed properties.
                // eslint-disable-next-line @typescript-eslint/no-explicit-any
                $state = reducers[chunk.action](state, chunk.param);
            }
            catch (error) {
                return controller.error(error);
            }
            if ($state !== state) {
                state = Object.freeze($state);
                controller.enqueue({
                    ...chunk,
                    state,
                });
            }
        },
    });
}
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