$.extend

 1 $.extend = function (target) {
 2             var deep, args = slice.call(arguments, 1)
 3             if (typeof target == ‘boolean‘) {
 4                 deep = target
 5                 target = args.shift()
 6             }
 7             args.forEach(function (arg) {
 8                 extend(target, arg, deep)
 9             })
10             return target
11         }
12 
13 function extend(target, source, deep) {
14             for (key in source)
15                 if (deep && (isPlainObject(source[key]) || isArray(source[key]))) {
16                     if (isPlainObject(source[key]) && !isPlainObject(target[key]))
17                         target[key] = {}
18                     if (isArray(source[key]) && !isArray(target[key]))
19                         target[key] = []
20                     extend(target[key], source[key], deep)
21                 }
22                 else if (source[key] !== undefined) target[key] = source[key]
23         }