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1.0.2 • Public • Published

Track selected changes to state

Monitors selected parts of a @watchable/store {@link Store}, triggering async callbacks.

@watchable/store-follow re-runs a Selector after each change to store state, notifying the follower callback whenever a different value is returned.

Read the API Reference or the reference usages below, or browse the source on Github.


Follow a Selector

// given this example store
const gameStore = createStore({
  steps: 0,
  direction: null,

// queue any changes to `steps` to be passed to the follower callback
  (state) => state.steps,
  async (steps) => {
    stepDisplay.innerText = `Completed ${steps} steps`;

Getting Started


npm install @watchable/store-follow

Advanced Usage

Explicitly handle queue.receive()

For complex examples needing access to underlying queue logic, use withSelectorQueue. It's what followSelector uses under the hood.

Sometimes you can't afford the syntactic sugar of followSelector which subscribes your callback automatically and hides the queue.receive() API that is notified of changes to your selection.

Like followSelector, withSelectorQueue also creates and subscribes a Queue to be notified every time a new value is returned, but it passes this Queue direct to your handler along with the initial selected value. It unsubscribes and disposes the queue only when your handler returns.


The withSelectorQueue example below needs direct access to queue.receive() as it waits for the first event of either...

  1. game character direction changed (from users keyboard input)
  2. timer expired (the character steps every 300ms)

It therefore has to use Promise.race() to handle either the receive or the timeout, whichever comes first.

// given this example store
const gameStore = createStore({
  steps: 0,
  direction: null,

// track direction (set elsewhere in the app)
const lastDirection = await withSelectorQueue(
  (state) => state.direction,
  async function ({ receive }, initialDirection) {
    let direction = initialDirection;
    let directionPromise = null;
    let stepPromise = null;
    while (direction !== null) {
      // loop until player stopped moving
      directionPromise = directionPromise || receive();
      stepPromise = stepPromise || sleep(STEP_MS);
      const winner: string = await Promise.race([
        directionPromise.then(() => "direction"),
        stepPromise.then(() => "step"),
      if (winner === "direction") {
        direction = await directionPromise; // direction changed
        directionPromise = null; // dispose promise
      } else if (winner === "step") {
        stepDirection(direction); // time to step
        stepPromise = null; // dispose promise

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npm i @watchable/store-follow

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