Deep Merge JavaScript Objects

deep-merge.js

/**
 * @description Method to check if an item is an object. Date and Function are considered
 * an object, so if you need to exclude those, please update the method accordingly.
 * @param item - The item that needs to be checked
 * @return {Boolean} Whether or not @item is an object
 */
function isObject(item) {
    return (item === Object(item) && !Array.isArray(item));
}

/**
 * @description Method to deeply merge objects, if the same key holds arrays, then these arrays are concatenated and primitive duplicate values removed
 * @param {Object} target - The targeted object that needs to be merged with the supplied @sources
 * @param {Object} [sources] - The object(s) that need to be merged with the @target
 * @return {Object}
 */
function deepMerge(target, ...sources) {
    if (!sources) {
        return target;
    }

    let result = target;

    if (isObject(result)) {
        for (let i = 0, len = sources.length; i < len; i++) {
            const elm = sources[i];

            if (isObject(elm)) {
                for (const key in elm) {
                    if (elm.hasOwnProperty(key)) {
                        if (isObject(elm[key])) {
                            if (!result[key] || !isObject(result[key])) {
                                result[key] = {};
                            }
                            deepMerge(result[key], elm[key]);
                        } else {
                            if (Array.isArray(result[key]) && Array.isArray(elm[key])) {
                                result[key] = Array.from(new Set(result[key].concat(elm[key])));
                            } else {
                                result[key] = elm[key];
                            }
                        }
                    }
                }
            }
        }
    }

    return result;
}