1 00:00:00,400 --> 00:00:01,670 Well welcome back. 2 00:00:02,040 --> 00:00:07,050 So the next few videos I'll cover a really important topic which is how do we make decisions in our 3 00:00:07,050 --> 00:00:08,260 Javascript code. 4 00:00:08,640 --> 00:00:11,960 And the first step is learning about something called boolean logic. 5 00:00:12,300 --> 00:00:17,160 So if you can recall back to the first few videos we talked about a data type in javascript called billions 6 00:00:17,640 --> 00:00:19,110 and billions are very simple. 7 00:00:19,110 --> 00:00:21,680 There's only two possible options for a boolean value. 8 00:00:21,810 --> 00:00:23,420 True or false. 9 00:00:23,760 --> 00:00:29,380 So boolean logic is simply writing statements that evaluate to be true or false. 10 00:00:29,610 --> 00:00:34,800 And eventually we'll use these statements to actually run certain parts of our code if something is 11 00:00:34,800 --> 00:00:38,610 true and some other parts of our code if something else is false. 12 00:00:38,640 --> 00:00:40,590 So these statements can be things like. 13 00:00:40,830 --> 00:00:42,130 A user is logged in. 14 00:00:42,240 --> 00:00:49,160 That's true or false but it could also be something as simple as if subtotal is greater than $10000 15 00:00:49,210 --> 00:00:49,480 . 16 00:00:49,770 --> 00:00:51,790 And those of value to be true or false. 17 00:00:51,810 --> 00:00:54,420 And we do something with them in our code. 18 00:00:55,080 --> 00:01:00,450 So the first thing we have to start with are comparison operators and comparison operators are things 19 00:01:00,450 --> 00:01:03,800 that most of us use all the time in our day to day life. 20 00:01:03,930 --> 00:01:08,490 It's how we compare two values to one another and it's how javascript does the same thing. 21 00:01:08,490 --> 00:01:12,050 So this chart here goes over all the comparison operators. 22 00:01:12,210 --> 00:01:17,400 It assumes that x is equal to 5 and you can plug in five wherever you see X.. 23 00:01:17,460 --> 00:01:21,120 So let's start with the simplest one greater then. 24 00:01:21,360 --> 00:01:25,350 So x is greater than 10 is five greater than 10. 25 00:01:25,560 --> 00:01:26,190 No. 26 00:01:26,370 --> 00:01:29,500 So that evaluates to be false. 27 00:01:29,850 --> 00:01:33,980 Next is greater than or equal to five greater than or equal to five. 28 00:01:34,110 --> 00:01:35,560 That is true. 29 00:01:36,120 --> 00:01:39,000 Next we have less than and less than or equal to. 30 00:01:39,120 --> 00:01:45,040 You can see that Plug-In 5 5 is not less than negative 50 so it evaluates to be false. 31 00:01:45,060 --> 00:01:47,870 Five is less than or equal to 100. 32 00:01:47,880 --> 00:01:49,600 So this evaluates to be true. 33 00:01:50,370 --> 00:01:52,560 The next four are a little bit different. 34 00:01:52,620 --> 00:01:55,350 They cover equality and non equality. 35 00:01:55,350 --> 00:02:03,120 So it's how we check if X is exactly equal to the number 5 or x is equal to that string 5 or X is anything 36 00:02:03,120 --> 00:02:05,000 but the number 20. 37 00:02:05,040 --> 00:02:08,460 It's how we basically check for equality. 38 00:02:08,460 --> 00:02:12,130 One thing you'll notice though is that there are two ways to check for equality. 39 00:02:12,150 --> 00:02:16,610 So there's one with two equal signs and one with three equal signs. 40 00:02:16,620 --> 00:02:19,340 Likewise there are two ways to check for quality. 41 00:02:19,620 --> 00:02:25,320 One with one equal sign and an exclamation point and another one with an exclamation point and two equal 42 00:02:25,320 --> 00:02:26,220 signs. 43 00:02:26,340 --> 00:02:28,920 So they both kind of go together in pairs. 44 00:02:29,100 --> 00:02:31,890 We'll start by talking about double equals here. 45 00:02:31,890 --> 00:02:35,180 Notice that X double equals the string five. 46 00:02:35,280 --> 00:02:40,770 So X is number five x double equals the string five is true. 47 00:02:41,670 --> 00:02:46,560 But if we do X triple the cross the string five that's false. 48 00:02:46,560 --> 00:02:52,230 And the reason for that is on my next slide here it has to do with something called type coercion. 49 00:02:52,260 --> 00:02:55,770 So when we use double equals it performs type coercion. 50 00:02:55,770 --> 00:03:01,440 And what that means is it basically takes the two numbers or the two strings or the two variables whatever 51 00:03:01,440 --> 00:03:02,070 they are. 52 00:03:02,280 --> 00:03:06,100 And it tries to turn them into a similar type so that I can compare them. 53 00:03:06,300 --> 00:03:09,980 So in this example x is in number 99. 54 00:03:10,170 --> 00:03:16,890 And when we double equals x with the string ninety nine that is true because javascript performs type 55 00:03:16,890 --> 00:03:17,420 coercion. 56 00:03:17,430 --> 00:03:23,850 It tries to get them to be the same format and then compare the value versus triple equals does not 57 00:03:23,850 --> 00:03:27,060 perform type scores and it cares not only about value. 58 00:03:27,090 --> 00:03:33,090 So not only that they both say 99 but that one is a string and one is a number which is not valid triple 59 00:03:33,090 --> 00:03:33,770 equals. 60 00:03:34,020 --> 00:03:37,880 So as a rule of thumb you should always use triple equals it's much safer. 61 00:03:37,890 --> 00:03:45,120 It's much more specific and it's not that as another example you can see here if we have no bar Y is 62 00:03:45,120 --> 00:03:51,750 equal to no no double equals undefined it's actually true even though they are very different values 63 00:03:51,750 --> 00:03:51,780 . 64 00:03:51,780 --> 00:03:53,200 They're not the same thing. 65 00:03:53,280 --> 00:03:58,930 Double equals considers that to be true and triple equals considers that to be false. 66 00:03:58,950 --> 00:04:04,260 So the last thing that I want to do here is show a few interesting cases of using double equals. 67 00:04:04,320 --> 00:04:08,570 And it basically behaves in ways you definitely would not expect. 68 00:04:08,580 --> 00:04:11,120 So this is just another warning of why you should not use it. 69 00:04:11,250 --> 00:04:14,680 But it also reveals a few of the underlying quirks in Javascript. 70 00:04:14,970 --> 00:04:21,890 So we can take the number excuse me the boolean true and double equals the string 1. 71 00:04:22,140 --> 00:04:23,780 And that's actually true. 72 00:04:24,210 --> 00:04:26,920 However if we do that with any other string number. 73 00:04:26,940 --> 00:04:28,290 So if I say true. 74 00:04:28,470 --> 00:04:30,110 So just to show you two equals 1. 75 00:04:30,180 --> 00:04:37,030 That is true but true equals 12 is false. 76 00:04:37,050 --> 00:04:41,510 Likewise I say zero doubles equals false. 77 00:04:42,070 --> 00:04:43,350 And that is true. 78 00:04:43,710 --> 00:04:46,640 No double equals undefined as we just saw is true. 79 00:04:46,830 --> 00:04:50,150 And probably the weirdest one is not a number. 80 00:04:50,190 --> 00:04:57,120 Double equals not a number is actually false even though they look like exactly the same thing. 81 00:04:57,120 --> 00:04:58,980 It's one of the weirdest quirks in javascript. 82 00:04:58,980 --> 00:05:05,920 The way that not numbers implemented it is actually not comparable to not a number. 83 00:05:06,090 --> 00:05:09,810 So the next thing that we're going to talk about is how do we actually change some of these together 84 00:05:09,870 --> 00:05:12,900 and make those more complex statements that we talked about earlier.