index.js

var hasOwn = Object.prototype.hasOwnProperty;
var toString = Object.prototype.toString;

function isPlainObject(obj) {
	if (!obj || toString.call(obj) !== '[object Object]' || obj.nodeType || obj.setInterval)
		return false;

	var has_own_constructor = hasOwn.call(obj, 'constructor');
	var has_is_property_of_method = hasOwn.call(obj.constructor.prototype, 'isPrototypeOf');
	// Not own constructor property must be Object
	if (obj.constructor && !has_own_constructor && !has_is_property_of_method)
		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 key === undefined || hasOwn.call( obj, key );
};

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

	// Handle a deep copy situation
	if ( typeof target === "boolean" ) {
		deep = target;
		target = arguments[1] || {};
		// skip the boolean and the target
		i = 2;
	}

	// Handle case when target is a string or something (possible in deep copy)
	if ( typeof target !== "object" && typeof target !== "function") {
		target = {};
	}

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

				// Prevent never-ending loop
				if ( target === copy ) {
					continue;
				}

				// Recurse if we're merging plain objects or arrays
				if ( deep && copy && ( isPlainObject(copy) || (copyIsArray = Array.isArray(copy)) ) ) {
					if ( copyIsArray ) {
						copyIsArray = false;
						clone = src && Array.isArray(src) ? src : [];

					} else {
						clone = src && isPlainObject(src) ? src : {};
					}

					// Never move original objects, clone them
					target[ name ] = extend( deep, clone, copy );

				// Don't bring in undefined values
				} else if ( copy !== undefined ) {
					target[ name ] = copy;
				}
			}
		}
	}

	// Return the modified object
	return target;
};