Deep Map Get/Set

deepMapGet.js

function deepMapGet(map, keys) {
    return keys.reduce(
        (resultValue, key) =>
            resultValue === undefined ? resultValue : resultValue.get(key),
        map,
    );
}

deepMapSet.js

function deepMapSet(map, keys, value) {
    const levels = keys.map(
        (key, index, { length: keyCount }) =>
            map.get(key) || (index < keyCount - 1 ? new Map() : undefined),
    );

    return keys.reduceRight(
        (result, key, index) =>
            new Map([...(index > 0 ? levels[index - 1] : map), [key, result]]),
        value,
    );
}