Svelte's Store implementation

svelte-store.js

function noop () {}

export function writable (init, start) {
  let subscribers = [];
  let prev = init;
  let running = false;
  let stop;

  function notify () {
    let subs = subscribers.concat();
    let value = prev;

    for (let subscriber of subs) {
      try {
        subscriber(value);
      } catch {}
    }
  }

  function set (curr) {
    if (!running) return;
    
    if (curr !== prev) {
      prev = curr;
      if (stop) notify();
    }
  }
  
  function update (updater) {
    set(updater(prev));
  }

  function subscribe (listener) {
    subscribers.push(listener);

    if (subscribers.length === 1) {
      running = true;
      prev = init;

      let ret = start(set);
      notify();

      stop = typeof ret === 'function' ? ret : noop;
    }

    return function unsubscribe () {
      let index = subscribers.findIndex((subscriber) => (
        subscriber === listener
      ));

      subscribers.splice(index, 1);

      if (subscribers.length === 0) {
        running = false;

        try {
          stop();
        } catch {}
      }
    }
  }

  return { update, set, subscribe };
}

export function readable (init, fn) {
  return {
    subscribe: writable(init, fn).subscribe,
  };
}

export function derived (store, fn, init) {
  return readable(init, (set) => {
    return store.subscribe((value) => {
      set(fn(value));
    });
  });
}