A tight functional-reactive update loop for Node.JS, inspired by Elm, Hyperapp, and Redux

app.js

const nodeEnv = {
  defer: (fn) => process.nextTick(fn),
};

const browserEnv = {
  defer: (fn) => setTimeout(fn, 0),
};

const create = (update, subscriptions = [], env = browserEnv) => {
  let state = update(undefined, {});
  const actionQueue = [];

  const waitForIdle = () => new Promise((resolve) => {
    if (actionQueue.length === 0) {
      resolve();
      return;
    }

    setTimeout(() => {
      waitForIdle().then(resolve);
    }, 1);
  });

  const nextAction = () => {
    if (actionQueue.length === 0) return;
    const messageAndParams = actionQueue.shift();
    state = update(state, messageAndParams, effect);
    env.defer(nextAction);
  };

  const effect = (message, params) => {
    const starved = actionQueue.length === 0;
    actionQueue.push({ message, params });
    if (starved) {
      nextAction();
    }
  };

  const getState = () => {
    return state;
  };

  const waitForState = (condition) => new Promise((resolve) => {
    if (condition(state)) {
      resolve(state);
      return;
    }
    setTimeout(() => waitForState(condition).then(resolve), 1);
  });


  const addSubscription = (subscription) => {
    subscription(effect);
  };

  subscriptions.forEach((subscription) => {
    addSubscription(subscription);
  });

  return {
    getState,
    waitForIdle,
    waitForState,
    effect,
  };
};

window.FrpApp = { create, nodeEnv, browserEnv }

example.js

const App = window.FrpApp;

const messagesReducer = (messages = [], { message, params }, trigger) => {
  switch (message) {
    case 'ADD_MESSAGE':
      return [...messages, params.message];
    case 'REMOVE_MESSAGES':
      return messages.filter(message => message!== params.message);
  }
}

const update = (state = {}, { message, params }, trigger) => {
  return {
    messages: messagesReducer(state.messages, { message, params }, trigger),
  }
};

const thatReallyBlanksMyBlankSub = count => (trigger) => {
  const fetchIt = (remaining) => {
    if (remaining === 0) return;
    fetch('https://api.chew.pro/trbmb')
      .then((response) => response.json())
      .then((quotes) => trigger('ADD_MESSAGE', { message: quotes[0] }))
      .then(() => {
        setTimeout(fetchIt, 10000, remaining - 1);
      });
  }
  
  fetchIt(count)
};

const instance = App.create(update, [thatReallyBlanksMyBlankSub(5)]);

instance
  .waitForState((state) => state.messages.length === 5)
  .then((state) => console.log('State snapshot', { ...state });// Should have 5 different That really _____s my ____. quotes