index.js

'use strict';

var hasOwn = Object.prototype.hasOwnProperty;
var toStr = Object.prototype.toString;
var defineProperty = Object.defineProperty;
var gOPD = Object.getOwnPropertyDescriptor;

var isArray = function isArray(arr) {
	if (typeof Array.isArray === 'function') {
		return Array.isArray(arr);
	}

	return toStr.call(arr) === '[object Array]';
};

var isPlainObject = function isPlainObject(obj) {
	if (!obj || toStr.call(obj) !== '[object Object]') {
		return false;
	}

	var hasOwnConstructor = hasOwn.call(obj, 'constructor');
	var hasIsPrototypeOf = obj.constructor && obj.constructor.prototype && hasOwn.call(obj.constructor.prototype, 'isPrototypeOf');
	// Not own constructor property must be Object
	if (obj.constructor && !hasOwnConstructor && !hasIsPrototypeOf) {
		return false;
	}

	// Own properties are enumerated firstly, so to speed up, if last one is own, then all properties are own.
	var key;
	for (key in obj) { /**/ }

	return typeof key === 'undefined' || hasOwn.call(obj, key);
};

// If name is '__proto__', and Object.defineProperty is available, define __proto__ as an own property on target
var setProperty = function setProperty(target, options) {
	if (defineProperty && options.name === '__proto__') {
		defineProperty(target, options.name, {
			enumerable: true,
			configurable: true,
			value: options.newValue,
			writable: true
		});
	} else {
		// eslint-disable-next-line no-param-reassign
		target[options.name] = options.newValue;
	}
};

// Return undefined instead of __proto__ if '__proto__' is not an own property
var getProperty = function getProperty(obj, name) {
	if (name === '__proto__') {
		if (!hasOwn.call(obj, name)) {
			return void 0;
		} else if (gOPD) {
			// In early versions of node, obj['__proto__'] is buggy when obj has __proto__ as an own property. Object.getOwnPropertyDescriptor() works.
			return gOPD(obj, name).value;
		}
	}

	return obj[name];
};

module.exports = function extend() {
	var options, name, src, copy, copyIsArray, clone;
	var target = arguments[0];
	var i = 1;
	var length = arguments.length;
	var deep = false;

	// Handle a deep copy situation
	if (typeof target === 'boolean') {
		deep = target;
		target = arguments[1] || {};
		// skip the boolean and the target
		i = 2;
	}
	if (target == null || (typeof target !== 'object' && typeof target !== 'function')) {
		target = {};
	}

	for (; i < length; ++i) {
		options = arguments[i];
		// Only deal with non-null/undefined values
		if (options != null) {
			// Extend the base object
			for (name in options) {
				src = getProperty(target, name);
				copy = getProperty(options, name);

				// Prevent never-ending loop
				if (target !== copy) {
					// Recurse if we're merging plain objects or arrays
					if (deep && copy && (isPlainObject(copy) || (copyIsArray = isArray(copy)))) {
						if (copyIsArray) {
							copyIsArray = false;
							clone = src && isArray(src) ? src : [];
						} else {
							clone = src && isPlainObject(src) ? src : {};
						}

						// Never move original objects, clone them
						setProperty(target, { name: name, newValue: extend(deep, clone, copy) });

					// Don't bring in undefined values
					} else if (typeof copy !== 'undefined') {
						setProperty(target, { name: name, newValue: copy });
					}
				}
			}
		}
	}

	// Return the modified object
	return target;
};