Deep merge two object together. Built this with AI and have no use for it anymore keeping around just in case.

deep-merge.js

/**
 * Deep merge two objects together keeping unique keys from each object.
 *
 * @param {object} mergeIntoObj - The object to merge another object into.
 * @param {object} mergeAndPrioritizeObj - The object to merge into mergeIntoObj. This object
 * has priority when collisions occur.
 * @returns
 */
const deepMerge(mergeIntoObj, mergeAndPrioritizeObj) => {

    const isPlainObject = (value) => value && typeof value === 'object' && value.constructor === Object;

    // Recursively merge plain objects and arrays.
    const merge = (a, b) => {
        for (const key in b) {
            if (isPlainObject(a[key]) && isPlainObject(b[key])) {
                merge(a[key], b[key]);
            } else {
                // eslint-disable-next-line no-param-reassign
                a[key] = b[key];
            }
        }
    };

    // Clone first object to target.
    const merged = { ...mergeIntoObj };

    // Merge objects recursively.
    merge(merged, mergeAndPrioritizeObj);

    return merged;
};