redux-sac/src/cow-value-model.js

function copyWith (obj, key, value) {
    const result = typeof key === 'number' ? obj == null ? [] : [...obj] : obj == null ? {} : {...obj};
    result[key] = value;
    return result;
}

const constructKeys = function (keyDescriptions) {
    const keys = [];

    function fillKeys (keyDescriptions) {
        keyDescriptions.forEach(each => {
            if (typeof each === 'number') keys.push(each);
            else if (Array.isArray(each)) fillKeys(each);
            else keys.push(...each.toString().split('.'));
        });
    }

    fillKeys(keyDescriptions);

    return keys;
};

const deepGet = (keys, obj) => keys.reduce((x, key) => x == null ? undefined : x[key], obj);

const deepPut = (keys, obj, val) => {
    function setVal (x, index) {
        if (index >= keys.length) return val;
        const key = keys[index],
            value = x == null ? undefined : x[key],
            modified = setVal(value, index + 1);
        return value === modified ? x : copyWith(x, key, modified);
    }

    return setVal(obj, 0);
};

export const deepGetOrNil = (...keyDescriptions) => {
    const keys = constructKeys(keyDescriptions);
    return obj => deepGet(keys, obj);
};

export const deget = deepGetOrNil;

export const deepCopyOnWrite = (...keyDescriptions) => {
    const keys = constructKeys(keyDescriptions);
    return val => obj => deepPut(keys, obj, val);
};

export const decow = deepCopyOnWrite;

export const cowValueModel = (...keyDescriptions) => {
    const keys = constructKeys(keyDescriptions);

    const GET_SENTINEL = {};

    return (obj, val = GET_SENTINEL) =>
        val === GET_SENTINEL ?
            deepGet(keys, obj) :
            deepPut(keys, obj, val);
};

export const cowWorkshop = (keys, fn = x => x) => (obj, {result = obj, resultKeys = keys, diff} = {}) => {
    keys.forEach((key, index) => {
        const value = fn(deget(key)(obj));
        if (typeof value === "undefined") return;
        const modifier = decow(resultKeys[index])(value);
        const oldResult = result;
        result = modifier(oldResult);
        if (result !== oldResult) {
            diff = modifier(diff);
        }
    });
    return {result, diff};
};