Original subtitles

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Now before continuing to learn JavaScript, 2

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in this new section, 3

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we should first activate a special mode in JavaScript, 4

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called Strict Mode. 5

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And here on my desktop I already have the folder, 6

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for this part 2 fundamental section. 7

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And for you as I explained before, 8

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these starred files come from the get up repertory 9

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that I showed you at the beginning of the course. 10

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So use that get up reparatory as your starter folder 11

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or ofcourse you can just copy that data 12

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somewhere else to your computer and work from there, 13

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whatever works for you. 14

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Now I'm creating here in UVS code window 15

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and lets open this new folder. 16

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So that's our new working folder 17

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and this time we already have the script file here 18

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and in the HDML file, 19

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we see that the script is also already linked, 20

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so we don't have to do that this time. 21

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At this point I assume that you already know how to do that. 22

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So closing the side bar here 23

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and lets now get started in this new section. 24

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And as I said lets start with strict mode. 25

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So strict mode is a special mode 26

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that we can activate in JavaScript, 27

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which makes it easier for us 28

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to write a secure JavaScript code. 29

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And all we have to do to activate strict mode 30

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is to write this ring at the beginning of the script 31

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so use strict and that's it. 32

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So with this, we activated strict mode 33

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for the entire script. 34

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Now what's important is that this line of code, 35

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so this statement here basically 36

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has to be the very first statement in the script. 37

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So if we have any code before this 38

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then strict mode will not be activated. 39

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Comments are allowed because JavaScript 40

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will just ignore them but no code. 41

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Okay, now we actually can also activate strict mode, 42

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only for a specific function or a specific block. 43

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But I don't really see the point in doing that 44

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and so I always just use it at the beginning of each script 45

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and so I believe that you should do the same. 46

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So always just put strict mode 47

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in the beginning of your scripts 48

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and like that write more secure code. 49

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And when secure, I mean that strict mode 50

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makes it easier for us developers 51

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to avoid accidental errors. 52

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So basically it helps us introduce the bugs into our code 53

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and that's because of 2 reasons. 54

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First, strict mode forbids us to do certain things 55

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and second, it will actually create visible errors for us 56

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in certain situations in which without strict mode 57

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JavaScript will simply fail silently 58

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without letting us know that we did a mistake. 59

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Okay. 60

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So again, first strict mode forbids us to do certain things 61

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and second it creates visible errors 62

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and the developer console, where in other situations 63

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JavaScript would just fail silently. 64

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Okay, but now to make this a bit more clear, 65

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Lets actually see an example of 66

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one of the most important changes 67

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that strict mode introduces. 68

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So lets create a variable here 69

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with a complicated name lets say hasDriverslicense 70

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and lets initialize it as false here. 71

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And then I'll also create, pass test and set that to true. 72

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And I'm using let in both of them 73

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because I want to change them later 74

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and actually this one not. 75

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So lets just use a const here, 76

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because remember by default we should always const 77

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to avoid accidental errors. 78

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So basically there is a certain person 79

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which right now does not have a drivers license 80

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but they just passed a test, 81

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that's why we have passed test set to true. 82

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And now we want to write some logic 83

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so that when they pass the test 84

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this variable here gets set to true. 85

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So, we can say if passTest, right? 86

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Because this one here is a boolean value 87

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and so this condition will be either true or false. 88

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And then lets introduce a simple bug here, okay? 89

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So this is a kind of hard to write variable name 90

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and so lets pretend that we are writing it wrong here. 91

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So hasDriversLicense equal to true. 92

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So what I did here was to basically omit the 's' 93

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So this letter here is not in this variable 94

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that am trying to change here to true. 95

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And again that is an error I'm making here on purpose, 96

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just to show you how strict mode can help us 97

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avoid a certain kind of error, 98

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because doing this is actually more common 99

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than you might think, okay? 100

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And now lets lock something to the console 101

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if the person has the drivers license. 102

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And so now were actually gonna go back 103

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to writing it correctly. 104

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So if hasDriversLicense and so you see that now 105

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this one is exactly the same, 106

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as VS code highlights both. 107

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So if hasDriversLicense then log to the console, 108

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I can drive. 109

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And now lets actually start 110

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by deactivating strict mode again, 111

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just so we see how this bug could affect our code. 112

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So I'm giving it a save and then lets go 113

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and open this index.hdml file. 114

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Okay? 115

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Now lets open up to console here 116

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and again I'm gonna increase the text 117

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by hitting command plus a couple of times here. 118

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Okay? 119

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So lets reload here just to make sure 120

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that everything is working 121

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and now what we see is that it didn't log I can drive 122

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to the console. 123

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Even though we had attempted to set 124

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hasDriversLicense to true here, 125

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so we expected that drivers license being true 126

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should then log I can drive. 127

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But it didn't work and that's because of the mistake 128

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that I just introduced here on purpose 129

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forgetting this 's' 130

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so without the error it should work, right? 131

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But without it, its not gonna work. 132

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And again because basically here 133

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I set a new variable to true 134

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and not the original one, 135

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so this one is still false 136

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and that's why here, this logging does not happen. 137

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So now our program has a bug 138

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which means that it has a mistake 139

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and we would now go ahead and find that bug. 140

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But, strict mode can actually help us with this. 141

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So lets turn it back on, 142

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give it a save, now try it again. 143

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And now we actually get an error. 144

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So here we see that hasDriversLicense is not defined, 145

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so in line number 6 which is right here 146

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and so this error message now tells us 147

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exactly what is wrong. 148

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So it tells us that this variable is not defined 149

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and then we can see, oh we did a mistake here, 150

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we're missing the 's', 151

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right, so now its the same variable, 152

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here, here and here, okay? 153

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And so if we try that now again. 154

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then the error is gone. 155

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And so strict mode really helped us avoid this kind of bug 156

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that we created here. 157

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You can experiment a little bit with this 158

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to make it really clear. 159

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Now another thing that strict mode does 160

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is to introduce a short list of variable names 161

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that are reserved for features 162

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that might be added to the language a bit later. 163

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For example, lets try to define a variable called interface, 164

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for example, for an audio interface 165

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and now if we try this. 166

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Then we see that, we get an unexpected strict mode 167

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reserved word, error message basically 168

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and that is again because JavaScript 169

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is reserving this word here for a feature 170

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that it might implement in the future. 171

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And so by reserving these kind of words 172

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it will then make it easier 173

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to implement the feature in the future. 174

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So another one is private for example, 175

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so const private, just some number here 176

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and so this one will also not work. 177

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And that's because in the future 178

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there might be something called a private field and classes 179

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or in some other places even in the language 180

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and they might want to use this word for that. 181

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And so strict mode reserves these words 182

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so we cannot use them for variables. 183

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It's the same logic as this one 184

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so we cannot call a variable 'if'. 185

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Right? 186

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And that's because there's already an if statement. 187

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So here we get an unexpected token 'if' 188

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so that's like the rules 189

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that I explained to you in the beginning 190

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about naming variables. 191

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Okay? Alright. 192

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Now there are actually a lot of other changes 193

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that affect things that we haven't learned about yet. 194

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So, that's stuff like functions, objects, 195

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setting properties on primitive values and many more. 196

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And I will mention them in future lectures 197

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when the time comes. 198

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But everything that I'm gonna say 199

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in the rest of the course from now on 200

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we'll always assume that you have strict mode turned on. 201

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And so I will add this line of code here 202

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to all the scripts in the future 203

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and so you should do the same. 204

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Anyway with this out of the way 205

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lets now start learning functions.