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

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

function copyMapWith (obj, key, value) {
    const result = obj == null ? new Map() : new Map(obj);
    result.set(key, value);
    return result;
}

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

const isMapKey = key => typeof key === 'object' && key.isKeyInMap === true;

export const keyInMap = key => ({isKeyInMap: true, key});

export const kim = keyInMap;

const copyWith = (obj, key, value) =>
    typeof key === 'number' ? copyArrayWith(obj, key, value) :
        isMapKey(key) ? copyMapWith(obj, key.key, value) :
            copyObjectWith(obj, key, value);

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

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

    fillKeys(keyDescriptions);

    return keys;
};

const getKey = (x, key) => x == null ? undefined : isMapKey(key) ? x.get(key.key) : x[key];

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

export const deget = deepGetOrNil;

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

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

    return val => obj => setVal(obj, 0, val);
};

export const decow = deepCopyOnWrite;

export const cowValueModel = (...keyDescriptions) => {
    const GET_SENTINEL = {};

    return (obj, val = GET_SENTINEL) =>
        (val === GET_SENTINEL ?
            deget(...keyDescriptions) :
            decow(...keyDescriptions)(val))
        (obj);
};

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};
};