1 00:00:00,720 --> 00:00:05,160 All right guys so the next pattern we're going to look at is the singleton pattern which is actually 2 00:00:05,160 --> 00:00:12,420 a manifestation of the module pattern a singleton object is an immediate anonymous function and it can 3 00:00:12,420 --> 00:00:16,170 only return one instance of an object at a time. 4 00:00:16,170 --> 00:00:21,720 All right so repeated calls to the constructor will just return the same instance and like the module 5 00:00:21,720 --> 00:00:27,110 pattern it maintains a private reference which nothing from the outside can access. 6 00:00:27,120 --> 00:00:33,360 Now an example of why you may want to use a singleton is maybe you only want for instance one user object 7 00:00:33,360 --> 00:00:36,540 created at a time maybe a logged in user. 8 00:00:36,580 --> 00:00:41,250 It would prevent you from having two users from being created at once. 9 00:00:41,250 --> 00:00:47,190 Now oftentimes singletons are frowned upon because they give you a global point of access rather than 10 00:00:47,190 --> 00:00:53,700 embracing encapsulation and that bothers a lot of programmers and that they can also be kind of hard 11 00:00:53,700 --> 00:00:55,230 to debug debug. 12 00:00:55,230 --> 00:00:59,360 I think singletons do have their place but I haven't used them too much. 13 00:00:59,370 --> 00:01:04,860 I actually don't have much experience with this pattern at all but I wanted to include it in this section 14 00:01:05,130 --> 00:01:08,530 because it is a well-known pattern in singletons. 15 00:01:08,530 --> 00:01:14,910 They don't just relate to javascript but too many programming languages and just programming in general. 16 00:01:14,910 --> 00:01:21,390 So let's go ahead and let me give you a simple example here so I just have a blank js file and we're 17 00:01:21,390 --> 00:01:30,660 going to create a variable called Singleton and we're going to set this to our if we so will put our 18 00:01:30,660 --> 00:01:31,400 function 19 00:01:35,070 --> 00:01:46,110 and let's create a variable called instance and we're going to have a function called create instance. 20 00:01:46,560 --> 00:01:53,400 This isn't a very practical example but like I said I don't have much experience with it so create an 21 00:01:53,400 --> 00:01:56,220 object variable and will set it to a new object 22 00:01:59,220 --> 00:02:07,620 and we'll just we'll just pass in a string here and we'll just say object instance 23 00:02:10,510 --> 00:02:14,630 paying them will return that object. 24 00:02:14,710 --> 00:02:19,520 All right so now down here we're going to have our return just like the module pattern that I showed 25 00:02:19,510 --> 00:02:26,490 you in the last video and we're going to have our Get instance function here. 26 00:02:29,930 --> 00:02:34,580 OK and then what we're going to do is we're going to say if there is not an instance 27 00:02:38,450 --> 00:02:45,290 then we want instance to equal create instance 28 00:02:47,870 --> 00:02:53,120 and then under the if statement we're going to return the instance. 29 00:02:53,360 --> 00:02:59,960 All right and then down here what I'm going to do is instantiate an object I'm going to call it instance 30 00:03:00,730 --> 00:03:12,910 a spell I write instance a equals and then will say Singleton Daut get instance. 31 00:03:13,480 --> 00:03:20,000 OK so if we go ahead and console the log 32 00:03:22,460 --> 00:03:29,720 instance a and save you see we're going to get a string which is a primitive and it's an art says object 33 00:03:29,720 --> 00:03:32,050 instance. 34 00:03:32,050 --> 00:03:33,620 And I mean this doesn't have to be a string. 35 00:03:33,620 --> 00:03:34,340 We could do 36 00:03:40,230 --> 00:03:41,830 we could do that. 37 00:03:41,960 --> 00:03:44,360 That probably makes more sense. 38 00:03:44,360 --> 00:03:53,470 All right so if I go here if I try and instantiate another object from the singleton. 39 00:03:53,510 --> 00:04:01,460 So if I say instance b equals Singleton Daut instance and then I compare these two. 40 00:04:01,490 --> 00:04:09,620 So it's actually seeing one comment that the console dog blog. 41 00:04:10,130 --> 00:04:11,200 And we're going to say 42 00:04:15,200 --> 00:04:24,500 instance a is equal to instance B and save and you see we get true. 43 00:04:24,500 --> 00:04:29,320 All right so we can never have more than one instance even if we try. 44 00:04:30,020 --> 00:04:34,240 It's just going to give us the back the same instance. 45 00:04:34,250 --> 00:04:34,550 All right. 46 00:04:34,550 --> 00:04:39,440 So that's going to be it I'm not going to go much further into this because like I said I don't have 47 00:04:39,440 --> 00:04:44,660 a lot of experience with this pattern and I don't want to give you any wrong information just know that 48 00:04:44,690 --> 00:04:48,490 you can't have more than one instance with this pattern. 49 00:04:48,500 --> 00:04:53,810 All right so in the next video guys we're going to take a look at the factory pattern which is a pattern 50 00:04:53,810 --> 00:04:57,780 used to create objects multiple objects.