1 00:00:01,300 --> 00:00:03,620 So, we talked about type conversion 2 00:00:03,620 --> 00:00:07,240 and coercion to numbers and two strengths, 3 00:00:07,240 --> 00:00:10,150 but I didn't mention booleans yet, right? 4 00:00:10,150 --> 00:00:12,360 That's because we need to learn the concept 5 00:00:12,360 --> 00:00:15,490 of truthy and falsy values first. 6 00:00:15,490 --> 00:00:17,703 And so, that's what we're gonna do now. 7 00:00:19,090 --> 00:00:21,240 So, let's get right into it. 8 00:00:21,240 --> 00:00:26,130 So, falsy values are values that are not exactly false, 9 00:00:26,130 --> 00:00:29,360 but will become false when we try to convert them 10 00:00:29,360 --> 00:00:31,150 into a boolean. 11 00:00:31,150 --> 00:00:34,970 And in JavaScript, there are only five falsy values. 12 00:00:34,970 --> 00:00:38,210 And let me actually write them down here for you. 13 00:00:38,210 --> 00:00:43,210 So, five falsy values and they are zero, the empty string, 14 00:00:47,050 --> 00:00:52,050 undefined, null, and not a number, and that's it. 15 00:00:53,020 --> 00:00:56,120 Of course, false itself is already false, 16 00:00:56,120 --> 00:01:00,070 so we don't need to include it in the list of falsy values. 17 00:01:00,070 --> 00:01:03,530 So again, all of these five values will be converted 18 00:01:03,530 --> 00:01:07,890 to false when we attempt to convert them to a boolean. 19 00:01:07,890 --> 00:01:09,940 They're not exactly false initially, 20 00:01:09,940 --> 00:01:14,000 but they will become when converted to a boolean. 21 00:01:14,000 --> 00:01:17,130 So, that's the definition of falsy values. 22 00:01:17,130 --> 00:01:20,500 Everything else are our so-called truthy values. 23 00:01:20,500 --> 00:01:23,860 So, values that will be converted to true, for example, 24 00:01:23,860 --> 00:01:27,025 any number that is not zero or any string that is not 25 00:01:27,025 --> 00:01:30,060 an empty string will be converted to true 26 00:01:30,060 --> 00:01:32,820 when we attempt to convert them to a boolean. 27 00:01:32,820 --> 00:01:36,023 And so, let's actually see that in practice. 28 00:01:37,270 --> 00:01:39,420 So, just like with numbers and strings, 29 00:01:39,420 --> 00:01:44,420 we can use the boolean function, boolean and then zero. 30 00:01:45,240 --> 00:01:50,240 And we know that this one is a falsy value, right? 31 00:01:50,530 --> 00:01:53,020 So, let's copy this and do this a couple of times 32 00:01:53,020 --> 00:01:54,533 for some different things. 33 00:01:55,470 --> 00:01:57,913 Let's do the same with undefined. 34 00:02:00,580 --> 00:02:05,580 Let's do it now with a string and it's not an empty string. 35 00:02:07,060 --> 00:02:10,750 And now, let's also do this with this empty object 36 00:02:10,750 --> 00:02:13,820 and we will learn about objects in the next section. 37 00:02:13,820 --> 00:02:16,660 So, this here is simply an empty object. 38 00:02:16,660 --> 00:02:19,453 So, let's see what the result of this is gonna be. 39 00:02:22,290 --> 00:02:25,460 And we see that indeed, converting to zero here 40 00:02:25,460 --> 00:02:29,000 to a boolean resulted in false, 41 00:02:29,000 --> 00:02:31,800 and the same for undefined, it's also false. 42 00:02:31,800 --> 00:02:35,770 But then the string, Jonas is true, 43 00:02:35,770 --> 00:02:38,730 and so that's why we say that any string that is not 44 00:02:38,730 --> 00:02:41,600 an empty string is a truthy value. 45 00:02:41,600 --> 00:02:45,000 And the same actually applies to this empty object here, 46 00:02:45,000 --> 00:02:47,420 it's also a truthy value. 47 00:02:47,420 --> 00:02:50,093 But if we tried to convert an empty string, 48 00:02:50,980 --> 00:02:53,630 well, I think then you can already know 49 00:02:53,630 --> 00:02:57,890 what the result will be, and it's gonna be false. 50 00:02:57,890 --> 00:03:00,810 Again because it's a falsy value, 51 00:03:00,810 --> 00:03:03,470 as we see up here in this list. 52 00:03:03,470 --> 00:03:06,270 So, that's what truthy and falsy values are. 53 00:03:06,270 --> 00:03:09,880 And this is how we could convert them to booleans. 54 00:03:09,880 --> 00:03:13,110 But in practice, I actually never did this in my life. 55 00:03:13,110 --> 00:03:16,200 I never used this function here in practice. 56 00:03:16,200 --> 00:03:18,690 This was just to show you the concept of truthy 57 00:03:18,690 --> 00:03:20,520 and falsely values. 58 00:03:20,520 --> 00:03:22,310 So in practice, the conversion 59 00:03:22,310 --> 00:03:26,320 to boolean is always implicit, not explicit, 60 00:03:26,320 --> 00:03:29,380 or in other words is always typed coercion 61 00:03:29,380 --> 00:03:32,640 that JavaScript does automatically behind the scenes. 62 00:03:32,640 --> 00:03:36,400 But when exactly does JavaScript do type coercion 63 00:03:36,400 --> 00:03:37,700 to booleans? 64 00:03:37,700 --> 00:03:40,410 Well, it happens in two scenarios. 65 00:03:40,410 --> 00:03:44,420 First, when using logical operators, and second in 66 00:03:44,420 --> 00:03:46,830 a logical context, like for example, 67 00:03:46,830 --> 00:03:50,070 in the condition of an if else statement. 68 00:03:50,070 --> 00:03:52,410 And we're gonna talk about logical operators in 69 00:03:52,410 --> 00:03:54,920 a future lecture, so let's now go back 70 00:03:54,920 --> 00:03:58,760 to the if else statement and see how type coercion works 71 00:03:58,760 --> 00:04:00,890 in the if else condition. 72 00:04:00,890 --> 00:04:04,540 So, let's say that we have a money variable 73 00:04:06,810 --> 00:04:09,460 and in the beginning we don't have any money. 74 00:04:09,460 --> 00:04:12,150 So, we set it to zero here. 75 00:04:12,150 --> 00:04:14,890 And now we can use what we just learned to test 76 00:04:14,890 --> 00:04:18,830 if the person has currently any money or not, right? 77 00:04:18,830 --> 00:04:20,540 So, let me actually write it first, 78 00:04:20,540 --> 00:04:23,550 and then I'm gonna explain to you how it works. 79 00:04:23,550 --> 00:04:28,063 So, if money then let's log to the console, 80 00:04:31,890 --> 00:04:34,463 something like don't spend it all 81 00:04:38,870 --> 00:04:41,590 and notice how I used the double quotes, 82 00:04:41,590 --> 00:04:44,313 because in here I have a single quote for the don't. 83 00:04:45,550 --> 00:04:47,450 And then we can also do an else block, 84 00:04:49,820 --> 00:04:53,540 so for the case that the person does not have any money, 85 00:04:53,540 --> 00:04:58,540 then I can just say, you should get a job. 86 00:05:01,150 --> 00:05:02,590 Okay. 87 00:05:02,590 --> 00:05:05,630 So, let's see if this works, and then I'm gonna explain 88 00:05:05,630 --> 00:05:07,640 to you why it works. 89 00:05:07,640 --> 00:05:11,050 So, indeed we got the you should get a job, 90 00:05:11,050 --> 00:05:14,240 and so that's the else part here, right? 91 00:05:14,240 --> 00:05:18,350 So, this here is the code that got executed, but why? 92 00:05:18,350 --> 00:05:21,930 Well, we know that money is a number right now. 93 00:05:21,930 --> 00:05:25,410 And this number is zero, but in the logical context 94 00:05:25,410 --> 00:05:27,890 of an if else statement condition, 95 00:05:27,890 --> 00:05:30,300 so that's right here in these parenthesis, 96 00:05:30,300 --> 00:05:34,580 here JavaScript will try to coerce any value into a boolean. 97 00:05:34,580 --> 00:05:37,720 So, no matter what we put here, if it's not a boolean, 98 00:05:37,720 --> 00:05:40,980 JavaScript will try to convert it to a boolean. 99 00:05:40,980 --> 00:05:44,122 And that happens using the truthy and falsy value rules 100 00:05:44,122 --> 00:05:47,060 that we just discussed in the beginning. 101 00:05:47,060 --> 00:05:50,150 So, we know that money here is zero, 102 00:05:50,150 --> 00:05:53,200 but zero is a falsy value, right? 103 00:05:53,200 --> 00:05:56,550 And so, in this logical environment here in this condition, 104 00:05:56,550 --> 00:05:59,280 this number zero will be converted to false. 105 00:05:59,280 --> 00:06:01,570 Just like it happened here 106 00:06:01,570 --> 00:06:05,650 when we convert it manually zero to a boolean, right? 107 00:06:05,650 --> 00:06:09,510 So here, this conversion to boolean returned false. 108 00:06:09,510 --> 00:06:12,700 And here the exact same thing happens, money is zero, 109 00:06:12,700 --> 00:06:15,070 and so zero is converted to false 110 00:06:15,070 --> 00:06:16,810 because it's a falsy value. 111 00:06:16,810 --> 00:06:20,290 And as a result, the else block here is executed, 112 00:06:20,290 --> 00:06:23,940 and so on the console we see you should get a job. 113 00:06:23,940 --> 00:06:26,100 Now, if we change this to something else 114 00:06:26,100 --> 00:06:29,610 and it doesn't matter what number, let's say 100, 115 00:06:29,610 --> 00:06:32,760 then 100 is a truthy value, 116 00:06:32,760 --> 00:06:36,000 and so this means that this condition will be true. 117 00:06:36,000 --> 00:06:40,163 And so the if block will be executed, wanna see? 118 00:06:41,380 --> 00:06:42,670 Yes, indeed. 119 00:06:42,670 --> 00:06:44,873 Now it says don't spend it all. 120 00:06:45,810 --> 00:06:47,973 Great, I hope that made sense. 121 00:06:48,910 --> 00:06:51,940 And we can test this in another situation 122 00:06:51,940 --> 00:06:55,000 to make really sure that you understand it. 123 00:06:55,000 --> 00:06:58,390 So, another use case for this truthy and falsy values is 124 00:06:58,390 --> 00:07:02,510 to check if a variable is actually defined or not. 125 00:07:02,510 --> 00:07:05,250 And this might seem like a weird use case, 126 00:07:05,250 --> 00:07:07,980 but you will see later in the course that it actually makes 127 00:07:07,980 --> 00:07:10,080 a lot of sense sometimes to test 128 00:07:10,080 --> 00:07:13,120 if something actually exists or not. 129 00:07:13,120 --> 00:07:17,200 So, let's define a variable here as height, 130 00:07:17,200 --> 00:07:20,070 and then we can simply do this. 131 00:07:20,070 --> 00:07:25,070 If height, then let's run this code, YAY! Height is defined. 132 00:07:35,350 --> 00:07:40,350 But if not, well, then just say height is undefined. 133 00:07:44,540 --> 00:07:46,800 Okay, so let's see the result. 134 00:07:46,800 --> 00:07:49,850 And again, I will explain you why it works this way, 135 00:07:49,850 --> 00:07:52,860 and maybe you can already guess why it works this way. 136 00:07:52,860 --> 00:07:55,800 So, can you try to explain it to yourself maybe? 137 00:07:55,800 --> 00:07:57,000 That would be a good idea 138 00:07:57,000 --> 00:07:59,830 to make sure you actually got the concept. 139 00:07:59,830 --> 00:08:03,940 But anyway, we know that height is undefined 140 00:08:03,940 --> 00:08:05,570 in this moment, right? 141 00:08:05,570 --> 00:08:08,530 Because we didn't assign it any value yet. 142 00:08:08,530 --> 00:08:10,780 And we know from our little list here, 143 00:08:10,780 --> 00:08:13,410 that undefined is a falsy value. 144 00:08:13,410 --> 00:08:18,100 And therefore, just like before in the money here, 145 00:08:18,100 --> 00:08:21,150 this height variable here in this logical context, 146 00:08:21,150 --> 00:08:23,960 will automatically be converted to a boolean. 147 00:08:23,960 --> 00:08:26,890 And since height is undefined and undefined is 148 00:08:26,890 --> 00:08:31,320 a falsy value, height right here will be false. 149 00:08:31,320 --> 00:08:34,163 Therefore, this block here will be executed. 150 00:08:35,430 --> 00:08:39,660 But now if we assign something to it, no matter what it is, 151 00:08:39,660 --> 00:08:43,510 then we will get the other result. 152 00:08:43,510 --> 00:08:48,390 So, now it's true, and so now this block here is executed. 153 00:08:48,390 --> 00:08:52,270 However, with this, we can actually run into a problem. 154 00:08:52,270 --> 00:08:56,240 So, let's say that the height is zero for reason, 155 00:08:56,240 --> 00:08:58,620 and that's a perfectly valid number, 156 00:08:58,620 --> 00:09:01,810 but watch what happens when we run this code. 157 00:09:01,810 --> 00:09:04,860 Now we get again, height is undefined. 158 00:09:04,860 --> 00:09:07,030 So, why is that? 159 00:09:07,030 --> 00:09:09,220 And to help you know the answer by now, 160 00:09:09,220 --> 00:09:13,180 well it's because zero is also a falsy value. 161 00:09:13,180 --> 00:09:16,660 And so this one will also trigger this else block. 162 00:09:16,660 --> 00:09:19,440 But in this case, that's actually not what we want. 163 00:09:19,440 --> 00:09:21,210 So, this is kind of a bug. 164 00:09:21,210 --> 00:09:23,490 It's an error in our application 165 00:09:23,490 --> 00:09:25,420 because here in this if else statement, 166 00:09:25,420 --> 00:09:27,900 we didn't account for this scenario. 167 00:09:27,900 --> 00:09:29,420 We only accounted for the scenario 168 00:09:29,420 --> 00:09:32,330 that the height is either defined or not, 169 00:09:32,330 --> 00:09:36,210 but we didn't think of the height being zero, right? 170 00:09:36,210 --> 00:09:38,780 And so right now we get hightest undefined, 171 00:09:38,780 --> 00:09:40,890 even though that's not true. 172 00:09:40,890 --> 00:09:43,500 So, this is just to illustrate an example 173 00:09:43,500 --> 00:09:46,300 that there can be problems using this approach. 174 00:09:46,300 --> 00:09:49,280 However, we can fix this using logical operators, 175 00:09:49,280 --> 00:09:51,350 which is gonna be very interesting, 176 00:09:51,350 --> 00:09:53,610 and we will talk about that a little bit later 177 00:09:53,610 --> 00:09:55,220 in the section. 178 00:09:55,220 --> 00:09:57,440 But first in the next lecture, we need to talk 179 00:09:57,440 --> 00:10:01,203 about equality operators, so stay tuned for that.