ts-09-typed-store

1.000

8/8 tests· typing

Proposed solution

```typescript
type Action = { type: "inc" } | { type: "add"; by: number };

export function createStore<S, A>(
  reducer: (state: S, action: A) => S,
  initial: S,
): {
  getState(): S;
  dispatch(action: A): void;
  subscribe(fn: () => void): () => void;
} {
  let state: S = initial;
  let listeners: Array<() => void> = [];

  function getState(): S {
    return state;
  }

  function dispatch(action: A): void {
    state = reducer(state, action);
    listeners.forEach(fn => fn());
  }

  function subscribe(fn: () => void): () => void {
    listeners.push(fn);
    return () => {
      const index = listeners.indexOf(fn);
      if (index > -1) {
        listeners.splice(index, 1);
      }
    };
  }

  return { getState, dispatch, subscribe };
}
```

Test output (stdout)

TAP version 13
# Subtest: starts at the initial state
ok 1 - starts at the initial state
  ---
  duration_ms: 0.376714
  type: 'test'
  ...
# Subtest: dispatch updates state via the reducer
ok 2 - dispatch updates state via the reducer
  ---
  duration_ms: 0.079351
  type: 'test'
  ...
# Subtest: subscribers fire on every dispatch
ok 3 - subscribers fire on every dispatch
  ---
  duration_ms: 0.060811
  type: 'test'
  ...
# Subtest: multiple subscribers all fire
ok 4 - multiple subscribers all fire
  ---
  duration_ms: 0.341304
  type: 'test'
  ...
# Subtest: unsubscribe stops notifications
ok 5 - unsubscribe stops notifications
  ---
  duration_ms: 0.070721
  type: 'test'
  ...
# Subtest: unsubscribing twice is harmless
ok 6 - unsubscribing twice is harmless
  ---
  duration_ms: 0.084501
  type: 'test'
  ...
# Subtest: subscriber sees the updated state when notified
ok 7 - subscriber sees the updated state when notified
  ---
  duration_ms: 0.311813
  type: 'test'
  ...
# Subtest: works with an object state shape
ok 8 - works with an object state shape
  ---
  duration_ms: 0.083181
  type: 'test'
  ...
1..8
# tests 8
# suites 0
# pass 8
# fail 0
# cancelled 0
# skipped 0
# todo 0
# duration_ms 80.936645

System prompt

You are an expert programmer. Solve the task exactly as specified. Output your solution as fenced code blocks using the required file name(s) and the exact function/type signatures requested. Prefer correctness; do not include prose outside code unless asked.