Streaming Chat Store Reducer

A reducer is a pure function (state, action) => newState — the same shape Redux and useReducer use to make state changes predictable. Here you build the reducer behind a streaming chat: the assistant's reply does not arrive all at once, so the store has to open an empty message, append text deltas as they stream in, and finally mark it complete — all without ever mutating the previous state.

State is an array of messages, each { id, role, content, status }. Implement streamingChatStoreReducer(state, action) handling four actions:

• ADD_USER_MESSAGE { id, content } — append a user message with status: 'done'.
• START_ASSISTANT_MESSAGE { id } — append an assistant message with empty content and status: 'streaming'.
• APPEND_DELTA { id, delta } — append delta to that message's content.
• FINALIZE_MESSAGE { id } — set that message's status to 'done'.

Signature

type Message = { id: string; role: 'user' | 'assistant'; content: string; status: 'streaming' | 'done' };

function streamingChatStoreReducer(state: Message[] | undefined, action: { type: string; id?: string; content?: string; delta?: string }): Message[];

Examples

let s = streamingChatStoreReducer([], { type: 'START_ASSISTANT_MESSAGE', id: 'a1' });
// → [{ id: 'a1', role: 'assistant', content: '', status: 'streaming' }]
s = streamingChatStoreReducer(s, { type: 'APPEND_DELTA', id: 'a1', delta: 'Hi' });
// → [{ id: 'a1', role: 'assistant', content: 'Hi', status: 'streaming' }]

// A no-op action returns the exact same reference.
const state = [{ id: 'a1', role: 'assistant', content: 'x', status: 'done' }];
streamingChatStoreReducer(state, { type: 'UNKNOWN' }) === state; // true

Notes

• Never mutate — do not push onto the array or reassign a message's fields. Return new arrays and new objects for what changed.
• Structural sharing — messages you did not touch must keep their original reference in the returned array.
• No-ops return the same reference — an unknown action, a delta for a missing id, or starting a message whose id already exists should return state unchanged.
• Default state — when state is undefined, treat it as [].
• Out of scope — deleting messages, editing user text, error states, and streaming tool calls.