1 00:00:00,950 --> 00:00:06,390 Now that we have a good idea of how the first rule works when the keyword this is outside of a declared 2 00:00:06,420 --> 00:00:12,940 object let's see what happens when the key word this is inside of a declared object. 3 00:00:12,960 --> 00:00:18,600 The second rule which we call the implicit or object rule states that when the key word this is found 4 00:00:18,660 --> 00:00:26,120 inside of a declared object the value of the keyword this will always be the closest parent object that 5 00:00:26,130 --> 00:00:27,100 was a lot of words. 6 00:00:27,300 --> 00:00:30,640 Let's see what we mean by that by looking at the code on the slide. 7 00:00:30,660 --> 00:00:33,590 Here we have a Person object but the key is a first name. 8 00:00:33,630 --> 00:00:35,910 Say hi and determine context. 9 00:00:36,090 --> 00:00:41,350 Let's start with the say hi method inside of this function we're returning the value of the string. 10 00:00:41,350 --> 00:00:45,960 Hi concatenated with the value of the first name property on the keyword. 11 00:00:45,960 --> 00:00:52,650 This Since we see that the keyword this is inside of a declared object we look at the closest parent 12 00:00:52,740 --> 00:00:55,710 object to figure out the value of the keyword. 13 00:00:55,710 --> 00:01:02,640 This we can see that the closest parent object is the person variable which means that the value of 14 00:01:02,640 --> 00:01:03,210 the keyword. 15 00:01:03,210 --> 00:01:07,010 This is the person object. 16 00:01:07,020 --> 00:01:09,520 Now let's examine the determine context method. 17 00:01:09,540 --> 00:01:11,970 This function should return true or false. 18 00:01:11,970 --> 00:01:16,260 Notice that we're using triple equals which will always evaluate to a boolean. 19 00:01:16,260 --> 00:01:18,320 What do you think this function will return. 20 00:01:18,330 --> 00:01:19,940 Think about where you see the key word. 21 00:01:19,940 --> 00:01:24,290 This is it inside or outside of a declared object. 22 00:01:24,390 --> 00:01:29,850 We see that the keyword this is inside of the person object which means that it will equal the person 23 00:01:29,940 --> 00:01:30,780 object. 24 00:01:30,900 --> 00:01:35,390 So this expression should evaluate to true before moving on. 25 00:01:35,430 --> 00:01:41,100 Take a second to review this code and put it in the chrome console or text editor to see exactly what's 26 00:01:41,100 --> 00:01:41,840 happening. 27 00:01:42,200 --> 00:01:47,370 Now we said in the implicit rule that the value of the keyword this will always be the closest parent 28 00:01:47,400 --> 00:01:48,300 object. 29 00:01:48,510 --> 00:01:52,000 But let's see how things get a bit tricky with nested objects. 30 00:01:52,500 --> 00:01:53,940 Let's walk through this code. 31 00:01:53,970 --> 00:01:58,960 Here we see that we have a person variable but this time the first name property is called. 32 00:01:59,040 --> 00:02:02,700 Once again we have a hi method and determine context method. 33 00:02:02,700 --> 00:02:06,570 Take a second and think about what those functions will return when called. 34 00:02:06,570 --> 00:02:10,770 Now notice we have an additional key in the person object called dog. 35 00:02:10,890 --> 00:02:14,110 The value of the donkey is actually another object. 36 00:02:14,190 --> 00:02:19,180 So let's examine the nested object inside of the dog object. 37 00:02:19,200 --> 00:02:24,360 We have a key of say hello which is a function that returns the string Hello joined with the key word 38 00:02:24,350 --> 00:02:24,470 . 39 00:02:24,480 --> 00:02:32,130 This using the two rules for determining the value of the keyword this pause the video and try to figure 40 00:02:32,130 --> 00:02:38,870 out what the value of the keyword this inside of the say hello function is. 41 00:02:38,880 --> 00:02:40,230 So would you come up with. 42 00:02:40,410 --> 00:02:44,230 What is the key word this refer to the person object the dog object. 43 00:02:44,280 --> 00:02:45,720 Or maybe the global object. 44 00:02:45,930 --> 00:02:51,090 Well let's go back to our first rule which states that if the keyword this is not inside of a declared 45 00:02:51,120 --> 00:02:53,820 object it will be the global object. 46 00:02:53,820 --> 00:02:58,520 In this case we see that the keyword this is inside of a declared object called Person. 47 00:02:58,770 --> 00:03:01,090 So we move on to the second rule. 48 00:03:01,170 --> 00:03:07,050 The second rule states that the value of the keyword this will always be the closest parent object. 49 00:03:07,050 --> 00:03:11,850 This is where things get a bit tricky even though the declared object is person. 50 00:03:11,940 --> 00:03:19,060 There is an object inside it called Dog which is the closest parent object to the say hello method. 51 00:03:19,110 --> 00:03:25,680 Now that we know that the keyword this inside of the say hello method refers to the dog object what 52 00:03:25,680 --> 00:03:27,530 is the value of this stuff. 53 00:03:27,540 --> 00:03:32,180 First name inside of the say hello method. 54 00:03:32,610 --> 00:03:37,870 Since the dog object does not have a key of first name the value will be undefined. 55 00:03:38,160 --> 00:03:42,230 So why don't we call the say hello method by typing person dog. 56 00:03:42,270 --> 00:03:44,830 Say hello we can see it returns. 57 00:03:44,830 --> 00:03:46,850 Hello undefined. 58 00:03:47,580 --> 00:03:54,750 We can also see that our determined context function inside of the dog object is returning false because 59 00:03:54,750 --> 00:04:02,450 the keyword this inside of the determine context function does not refer to the person object anymore 60 00:04:02,470 --> 00:04:03,190 . 61 00:04:04,050 --> 00:04:07,760 So how can we fix this or if we want our Say hello method to return. 62 00:04:07,770 --> 00:04:14,070 Hello cold instead of hello undefined you would need some way of explicitly changing the value of the 63 00:04:14,080 --> 00:04:21,840 keyword this and that is exactly what the call apply and bind functions can do for us in the next video 64 00:04:21,840 --> 00:04:22,120 . 65 00:04:22,140 --> 00:04:27,990 We'll see the third rule for determining the value of the keyword this using call apply and bind. 66 00:04:28,060 --> 00:04:29,190 Things are getting exciting. 67 00:04:29,190 --> 00:04:30,340 See you in the next video.