derive-zustand/dist/index.js

A function to create a derived Zustand store from stores

export function derive(deriveFn) {
    const listeners = new Set();
    const subscriptions = new Map();
    let state;
    let dependencies;
    let invalidated = true;
    const invalidate = () => {
        if (invalidated) {
            return;
        }
        invalidated = true;
        listeners.forEach((listener) => listener(state, state));
    };
    const getState = () => {
        if (!invalidated) {
            return state;
        }
        if (!dependencies ||
            Array.from(dependencies).some(([store, value]) => !Object.is(store.getState(), value))) {
            const newDependencies = new Map();
            const get = (store) => {
                if (!store) {
                    return state;
                }
                const s = store.getState();
                newDependencies.set(store, s);
                return s;
            };
            state = deriveFn(get);
            dependencies = newDependencies;
        }
        if (listeners.size) {
            const deps = new Set(dependencies.keys());
            subscriptions.forEach((unsubscribe, store) => {
                if (deps.has(store)) {
                    deps.delete(store);
                }
                else {
                    unsubscribe();
                    subscriptions.delete(store);
                }
            });
            deps.forEach((store) => {
                subscriptions.set(store, store.subscribe(invalidate));
            });
            invalidated = false;
        }
        return state;
    };
    const subscribe = (listener) => {
        listeners.add(listener);
        return () => {
            listeners.delete(listener);
            if (!listeners.size) {
                subscriptions.forEach((unsubscribe) => unsubscribe());
                subscriptions.clear();
                invalidated = true;
            }
        };
    };
    const store = {
        getState,
        subscribe,
        getInitialState: () => {
            throw new Error('getInitialState is not available in derived store');
        },
        setState: () => {
            throw new Error('setState is not available in derived store');
        },
        destroy: () => {
            throw new Error('destory is not available in derived store');
        },
    };
    return store;
}