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Welcome back in the previous video we learn that when the keyword this is not inside of a declared object

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its value is the global object.

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Let's quickly demonstrate what I mean by declared object in case the previous video still left you a

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bit confused.

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Remember the material here is quite challenging.

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So try watching these videos a couple of times and typing the code examples to reinforce your knowledge

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.

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I'm going to make an empty object called data.

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What I have just done is declared an object the data object did not exist until I declared it.

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Now I'm going to attach a property onto this object called instructor and set the value equal to LTE

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.

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Now our data object that we declared has a key with the value of elhi inside of it.

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This is what we mean when we say inside of a declared object.

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If that still confuses you that's all right.

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It will make even more sense when we examine the second rule and see the keyword this inside of an object

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.

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Now let's finish up with the first rule.

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As you can see here when we put the key word this inside of a function its value is still the global

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object.

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Remember we said that the global rule applies unless the keyword this is inside of a declared object

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but it's inside of a function right now.

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So when we call the function what is this the value returned is the window object.

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Now let's look at one final example.

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What happens if we attach properties to the keyword this when its value is the global object.

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And remember when I say Global Object I mean the window object in different javascript environments

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.

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This value will change.

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But since we're in the browser the global object is the window object.

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Now let's take a look at our variables in this function in the variables in this function.

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We are attaching a property onto the keyword this called Person and setting it equal to Ellie.

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Since the key word this refers to the global object anything we attach onto it it becomes a global variable

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which means we can use it outside of the function.

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This may seem like something we might want to do but this is actually very bad practice.

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Let's quickly revisit how variables and scope work in javascript.

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I'm going to declare a variable called dog and said the value equal to Rusty.

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I'm then going to make a function called meek person an inside declare a variable called person using

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the VAR keyword and said it equal to code.

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Since I declared this variable inside of the function and I am not returning it's value I do not have

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access to it outside of the function.

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However since I declared my dog variable in the global scope I have access to it everywhere.

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Now we can get around the problem of not being able to access our person variable by omitting the VAR

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keyword inside of our function.

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However this is very bad practice.

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It's generally best practice to declare all of our variables that we want to use in multiple functions

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at the top of our code even if they do not have a value and then assign those values at a later time

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in a function.

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Unfortunately since the keyword this is the global object it's quite easy to accidentally declare global

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variables inside of a function.

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Let's see an example.

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I'm going to make a function called mistake an inside set a property on the keyword this called bad

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idea to be equal to the string.

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Whoops.

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Now I'm going to call that function and we can see that I have access to my bad idea variable.

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I accidentally just made a global variable.

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Thankfully in yes 5 the previous version of javascript something called strict mode was added to help

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us.

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Javascript developers read better cope when strict mode is enabled.

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Which you can do by adding use strict in double quotes.

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The value of the keyword this when inside of a function is undefined.

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It's not the global object.

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This means that if we try to attach properties onto it we get a type error.

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Since we can't attach properties to undefined you might have seen this error quite a bit.

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This stops us from accidentally creating global variables and allows us to use javascript best practices

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.

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In this video we reviewed the first rule for determining the keyword this and saw how we can accidentally

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create global variables inside of functions.

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We also saw how strict mode can prevent us from making this mistake in the next video.

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We'll be discussing the second rule and what the value of the keyword this is when it is inside of a

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declared object see there