An object merge algorithm. Doesn't boast performance at all, but will properly merge nested objects and arrays (by concatenation). This only works if the property is the same type on both objects, otherwise the second object's property will be taken and used to overwrite the target property in the merged object.

merge.js

module.exports = function merge(a, b) {
    return (Object.entries(a).map(e => ({
        entries: e,
        src: a,
        other: b
    })).concat(Object.entries(b).map(e => ({
        entries: e,
        src: b,
        other: a
    }))
    )).reduce((merged, { entries: [key, value], src: a, other: b }) => {
        if (!merged[key])
            if (value instanceof Array && b[key] instanceof Array) {
                merged[key] = [a[key], b[key]].flat();
            } else if (value instanceof Object && b[key] instanceof Object) {
                merged[key] = merge(value, b[key]);
            } else merged[key] = a[key] || b[key];
        return merged;
    }, {});
}