mergeObjects.js

function uniqueReducer(unique, item) {
    return unique.includes(item) ? unique : [...unique, item];
}

function mergeObjectsReducer(accumulator, current) {
    return {
        ...accumulator,
        ...Object.keys(current)
            .filter(
                key =>
                    accumulator[key] === undefined ||
                    (Array.isArray(accumulator[key]) && Array.isArray(current[key])),
            )
            .reduce(
                (object, key) => ({
                    ...object,
                    [key]: [...(accumulator[key] || []), ...current[key]].reduce(uniqueReducer, []),
                }),
                {},
            ),
    };
}

var x = [{a: [1,2,3]}, {a: [3,4,5]}, {b: [99]}].reduce(mergeObjectsReducer, {});

console.log(x);