Build a Minimal createStore() (Redux-Lite: dispatch, subscribe, reducers)

Q: Build a Minimal createStore() (Redux-Lite: dispatch, subscribe, reducers)

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** all 3 real behaviors — correct reducer-driven state updates, subscriber notification on every real dispatch, and a genuinely working unsubscribe that stops FUTURE notifications while state updates continue normally — were verified directly through a real dispatch sequence. ## 1. The problem, restated A minimal store holds a single piece of state, updated ONLY by passing an `action` object through a pure `reducer` function via `dispatch`; any number of `subscribe`d listener functions are notified after every real dispatch, and `subscribe` itself returns an unsubscribe function to later stop that SPECIFIC listener's notifications. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | subscribe returns an unsubscribe function? | Yes, the real, standard Redux convention — genuinely needed to correctly clean

The problem

Problem, as an interviewer would state it: "Implement a minimal createStore(reducer, initialState) — like Redux — with getState(), dispatch(action), and subscribe(listener)."

Examples:

Code

const store = createStore(counterReducer, { count: 0 });
store.subscribe(() => console.log(store.getState()));
store.dispatch({ type: "INCREMENT" }); // logs { count: 1 }

Clarifying questions expected:

Should subscribe return an unsubscribe function, and must calling it correctly stop future notifications for JUST that listener?
Should the reducer receive the CURRENT state and an action, returning a brand-new state (never mutating), matching the real, standard Redux convention?
Should dispatch itself return anything meaningful, like the action that was dispatched?

Code / implementation expected: Yes — real, direct proof of a real dispatch sequence, confirming correct state updates, subscriber notification on every dispatch, and a correctly-working unsubscribe.

state-management
system-design

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Medium

How to read this doc: all 3 real behaviors — correct reducer-driven state updates, subscriber notification on every real dispatch, and a genuinely working unsubscribe that stops FUTURE notifications while state updates continue normally — were verified directly through a real dispatch sequence.

1. The problem, restated

A minimal store holds a single piece of state, updated ONLY by passing an action object through a pure reducer function via dispatch; any number of subscribed listener functions are notified after every real dispatch, and subscribe itself returns an unsubscribe function to later stop that SPECIFIC listener's notifications.

2. Clarifying questions a strong candidate asks

Question	Why it matters
subscribe returns an un

The solution is hidden so you can try first

7 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: dispatch correctly updates state and notifies subscribers, and unsubscribe genuinely stops further notifications for that listener