Flow is useful when an app needs signal-like state, event handlers, async signals, and small workflow helpers without adopting a full statechart engine.

Pick the smallest layer that solves the problem:

L1 primitives: use createSignal, createComputed, createAsyncSignal, and createStore when an adapter or library needs explicit values and controllers.
L2 Flow: use flow(...) when state changes should run through named events and batched plain functions.
L2.5 composition: use compose(...) and parallel(...) when a Flow handler needs ordered or fan-out/fan-in work without a full helper vocabulary.
L3 steps: use set(...), when(...), branch(...), dispatch(...), and after(...) when repeated workflow wiring should read as reusable steps.

Install

pnpm add @async/flow

Quick Start

import { flow, status } from "@async/flow";

const counter = flow({
  store: {
    count: 0,
    phase: status("idle", ["idle", "active"])
  },

  on: {
    increment(store, input = {}) {
      store.count += input.by ?? 1;
      store.phase = "active";
    },

    reset(store) {
      store.count = 0;
      store.phase = "idle";
    }
  }
});

counter.dispatch("increment", { by: 2 });

counter.count; // 2
counter.phase; // "active"

A Flow instance combines:

store: author-facing values with getter/setter behavior.
_: non-enumerable internal controller namespace for _ store fields.
dispatch(name, input): event execution.
can(name, input?): event availability checks.
explain(name, input?): structured blocked-event reasons.
describe(): public inspection metadata for stores, handlers, transitions, and guards.

The package also provides compose(...), parallel(...), and remember(...) for ordered handler steps.

Store Values

Plain primitives and arrays become writable store values. Computed values are read-only. Plain record values stay explicit; use signal(value) when an object should be a single writable value.

import { computed, flow, signal, status } from "@async/flow";

const cart = flow({
  store: {
    items: [],
    settings: signal({ currency: "USD" }),
    count: computed(function () {
      return this.items.length;
    }),
    isEmpty: computed(function () {
      return this.count === 0;
    }),
    phase: status("idle", ["idle", "ready"])
  },

  on: {
    add(store, input) {
      store.items = [...store.items, input.item];
      store.phase = "ready";
    }
  }
});

cart.dispatch("add", { item: { id: "sku_123" } });

cart.count; // 1
cart.items; // [{ id: "sku_123" }]
cart.settings = { currency: "EUR" };

Computed function callbacks read store values directly from this.

Async Signals

asyncSignal(loader) declares a lazy async value with lifecycle state and explicit controls. Loaders read Flow store data through this.store; lifecycle tools are available through the function receiver.

import { asyncSignal, flow } from "@async/flow";

const greeting = flow({
  store: {
    name: "World",
    _request: asyncSignal(async function () {
      const response = await fetch(`/api/greeting/${this.store.name}`, {
        signal: this.signal
      });
      return response.text();
    }),
    get status() {
      return this._request.status;
    },
    get value() {
      return this._request.get();
    }
  },

  on: {
    fetch() {
      return this.store._request.load();
    },

    reload() {
      return this.store._request.reload();
    },

    cancel(_store, reason) {
      return this.store._request.cancel(reason);
    }
  }
});

await greeting.fetch();

greeting.value; // loaded text
greeting.status; // "ready"

Lazy and immediate async signals can both use internal fields starting with _ for controller methods while exposing public getters as normal Flow values.

const profile = flow({
  store: {
    _user: asyncSignal({ immediate: true }, async function () {
      const response = await fetch("/api/user", { signal: this.signal });
      return response.json();
    }),
    get user() {
      return this._user.get();
    },
    get status() {
      return this._user.status;
    }
  },

  on: {
    reloadUser() {
      return this.store._user.reload();
    }
  }
});

profile.user; // current value
profile.status; // "loading", "ready", or "error"

More detail: Async Signal Lifecycle.

Compose And Step Workflows

Use compose(...) for ordered steps that should share one Flow handler input. Each step receives (store, input, previous). Use parallel(...) when one ordered step should run independent effects before continuing. Use root-exported step helpers when the repeated parts are store writes, gates, branches, event dispatches, or scheduled follow-up events.

import { compose, every, flow, matches, not, parallel, set, status, when } from "@async/flow";

const checkout = flow({
  store: {
    step: status("shipping", ["shipping", "payment", "review"]),
    canSubmit: true,
    readyToSubmit: every(matches("step", "review"), (store) => store.canSubmit),
    blocked: not((store) => store.readyToSubmit),
    loading: false,
    orderId: null
  },

  on: {
    submit: compose([
      when((store) => store.readyToSubmit),
      set("loading", true),
      parallel({
        inventory(_store, input) {
          return reserveInventory(input.form);
        },
        tax(_store, input) {
          return calculateTax(input.form);
        }
      }),
      async (_store, input) => {
        const order = await submitOrder(input.form);
        return order.id;
      },
      (store, _input, orderId) => {
        store.orderId = orderId;
      },
      set("loading", false)
    ])
  }
});

compose stays synchronous until a step returns a promise-like value. Flow then flushes the current synchronous batch and resumes later steps in a fresh batch. That lets loading = true render before async work settles.

More detail: Compose And Status Helpers.

Event Availability And Inspection

Flow can answer whether an event is callable now without dispatching it.

checkout.can("submit"); // true for this plain composed handler
checkout.explain("missing");
// { event: "missing", allowed: false, reason: "unknown_event" }

Use describe() when adapters or tests need stable public metadata:

const description = checkout.describe();

description.handlers; // ["submit"]
description.store.step.kind; // "status"

Descriptions expose names, current values, lifecycle state, and safe metadata. They do not expose raw handlers or predicates.

Runtime Options

The top-level authoring helper accepts either config or options plus config.

flow(config);
flow({ scheduler, context }, config);

With two arguments, the first object is always runtime options and the second is always Flow config.

Handlers receive (store, input). Runtime capabilities are available through method syntax or normal functions:

const appFlow = flow(
  {
    context() {
      return { logger: console };
    }
  },
  {
    store: {
      count: 0
    },

    on: {
      increment(store, input) {
        store.count += input.by;
        this.logger.log(store.count);
        return this.dispatch("read");
      },

      read(store) {
        return store.count;
      }
    }
  }
);

Receiver capabilities include this.store, this.dispatch(name, input), this.can(name, input), this.explain(name, input), this.describe(), this.after(ms, eventName, input), and this.dispose(cleanup).

Root And Subpaths

The root package exports the complete opinionated Flow surface. Use subpaths when a consumer wants a narrower entrypoint.

import {
  after,
  asyncSignal,
  bool,
  branch,
  compose,
  computed,
  createAsyncSignal,
  createFlow,
  createSignal,
  createStore,
  defineAsyncSignal,
  defineFlow,
  dispatch,
  every,
  flow,
  matches,
  not,
  parallel,
  remember,
  set,
  signal,
  some,
  status,
  when
} from "@async/flow";

The old @async/flow/run subpath is not public. Use the root compose(...) export or the narrow @async/flow/compose entrypoint.

Docs

Package Checks

pnpm test
pnpm run typecheck
pnpm run pack:check

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Package last updated on 23 Jun 2026

@async/flow