index.js

'use strict';
const isOptionObject = require('is-plain-obj');

const {hasOwnProperty} = Object.prototype;
const {propertyIsEnumerable} = Object;
const defineProperty = (object, name, value) => Object.defineProperty(object, name, {
	value,
	writable: true,
	enumerable: true,
	configurable: true
});

const globalThis = this;
const defaultMergeOptions = {
	concatArrays: false,
	ignoreUndefined: false
};

const getEnumerableOwnPropertyKeys = value => {
	const keys = [];

	for (const key in value) {
		if (hasOwnProperty.call(value, key)) {
			keys.push(key);
		}
	}

	/* istanbul ignore else  */
	if (Object.getOwnPropertySymbols) {
		const symbols = Object.getOwnPropertySymbols(value);

		for (const symbol of symbols) {
			if (propertyIsEnumerable.call(value, symbol)) {
				keys.push(symbol);
			}
		}
	}

	return keys;
};

function clone(value) {
	if (Array.isArray(value)) {
		return cloneArray(value);
	}

	if (isOptionObject(value)) {
		return cloneOptionObject(value);
	}

	return value;
}

function cloneArray(array) {
	const result = array.slice(0, 0);

	getEnumerableOwnPropertyKeys(array).forEach(key => {
		defineProperty(result, key, clone(array[key]));
	});

	return result;
}

function cloneOptionObject(object) {
	const result = Object.getPrototypeOf(object) === null ? Object.create(null) : {};

	getEnumerableOwnPropertyKeys(object).forEach(key => {
		defineProperty(result, key, clone(object[key]));
	});

	return result;
}

/**
 * @param {*} merged already cloned
 * @param {*} source something to merge
 * @param {string[]} keys keys to merge
 * @param {Object} config Config Object
 * @returns {*} cloned Object
 */
const mergeKeys = (merged, source, keys, config) => {
	keys.forEach(key => {
		if (typeof source[key] === 'undefined' && config.ignoreUndefined) {
			return;
		}

		// Do not recurse into prototype chain of merged
		if (key in merged && merged[key] !== Object.getPrototypeOf(merged)) {
			defineProperty(merged, key, merge(merged[key], source[key], config));
		} else {
			defineProperty(merged, key, clone(source[key]));
		}
	});

	return merged;
};

/**
 * @param {*} merged already cloned
 * @param {*} source something to merge
 * @param {Object} config Config Object
 * @returns {*} cloned Object
 *
 * see [Array.prototype.concat ( ...arguments )](http://www.ecma-international.org/ecma-262/6.0/#sec-array.prototype.concat)
 */
const concatArrays = (merged, source, config) => {
	let result = merged.slice(0, 0);
	let resultIndex = 0;

	[merged, source].forEach(array => {
		const indices = [];

		// `result.concat(array)` with cloning
		for (let k = 0; k < array.length; k++) {
			if (!hasOwnProperty.call(array, k)) {
				continue;
			}

			indices.push(String(k));

			if (array === merged) {
				// Already cloned
				defineProperty(result, resultIndex++, array[k]);
			} else {
				defineProperty(result, resultIndex++, clone(array[k]));
			}
		}

		// Merge non-index keys
		result = mergeKeys(result, array, getEnumerableOwnPropertyKeys(array).filter(key => !indices.includes(key)), config);
	});

	return result;
};

/**
 * @param {*} merged already cloned
 * @param {*} source something to merge
 * @param {Object} config Config Object
 * @returns {*} cloned Object
 */
function merge(merged, source, config) {
	if (config.concatArrays && Array.isArray(merged) && Array.isArray(source)) {
		return concatArrays(merged, source, config);
	}

	if (!isOptionObject(source) || !isOptionObject(merged)) {
		return clone(source);
	}

	return mergeKeys(merged, source, getEnumerableOwnPropertyKeys(source), config);
}

module.exports = function (...options) {
	const config = merge(clone(defaultMergeOptions), (this !== globalThis && this) || {}, defaultMergeOptions);
	let merged = {_: {}};

	for (const option of options) {
		if (option === undefined) {
			continue;
		}

		if (!isOptionObject(option)) {
			throw new TypeError('`' + option + '` is not an Option Object');
		}

		merged = merge(merged, {_: option}, config);
	}

	return merged._;
};