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Now that we explored try catch,

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I wanna dive into a totally different,

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yet slightly related concept.

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I wanna talk about scoping of variables,

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constants and functions.

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Now what's scoping?

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Scoping simply means that variables, constants and functions

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can only be used in certain places.

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Now for example, if I create a read file function

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in errors.js, I can only use it in that file,

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unless I export it with module exports.

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Then we can export an object

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and export our read file function like this if we want to.

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That's what you saw before in the course already.

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This is only the case in NodeJS, though.

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In browser side JavaScript,

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you can't use this module exports feature,

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but there you also don't need to,

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because in browser side JavaScript,

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if you simply have a couple of script tags in your HTML file

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that import different JavaScript files,

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the variables and functions defined in those files

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will be accessible in our files automatically.

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So that file-based scoping,

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locking in values into a file, is exclusive to NodeJS.

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But we also have different forms of scoping

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that apply to both the browser and NodeJS.

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And the most important aspect here

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or the most important thing to note about scoping here

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is that constants and functions and variables

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are only available in the block where you defined them.

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Now what's a block?

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A block in JavaScript is simply a section of code

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that's wrapped by curly braces,

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with the exception of the creation of an object.

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If you use curly braces on the right side of an equals sign

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to create an object,

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then in here, we don't have a block of code.

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Instead here, we're using curly braces to create an object.

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In all our places in the code,

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like here, when we define a function,

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or if we try and catch, or if we have a for loop,

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whenever we have curly braces here,

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these curly braces mark a block of code.

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A block of code that belongs to the loop,

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a block of code that belongs to this function,

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or a block of code that belongs to try, or catch.

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And if I define a constant here

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in this block here, for example,

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it's only available in this block.

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I can't console log file data here

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outside of the try block.

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If I try to do that,

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and I then execute this errors.json file,

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I get an error that file data is not defined.

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Because here in line 9, file data is totally unknown.

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The constant file data,

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which I create in the try block,

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is only available inside of that try block.

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So console logging it here would work

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if we wouldn't fail to read from the file system.

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Console logging it here fails.

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If we wanna access file data in both the try block

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and in this line,

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we instead have to create a variable file data

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in the function block,

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in which both the try block and line 10 are,

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and then instead of creating a new constant

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of that same name,

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we simply assign file data here in try.

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Because that's always important.

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When I say that variables, constants and functions

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are only available in the block in which they are defined,

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that also means that they are available

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in any nested blocks.

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So here, file data is defined

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in the read file function block.

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That also makes it available inside of the try block though,

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since the try block here is inside of that function block.

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So from outside to inside is supported,

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the other way around isn't.

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And this concept, which is called block scoping,

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is important to understand.

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It is important to understand

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that your constants and variables are only available

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inside of the block where you define them,

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not outside of that block.

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Now directly related to this idea

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of scoping variables and constants,

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we also have the concept

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of shadowing variables or constants.

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And you can see that here, if I actually re-add const

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in front of file data in line 6.

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Now file data already is a variable defined in line 4.

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And now instead of assigning to that existing variable,

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I am creating a brand-new constant.

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Now what will happen if we now try to execute this file?

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Let's give it a try.

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If I execute this, everything works,

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but please note that we get undefined here.

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And that's coming from line 10 actually.

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Now we're getting undefined here

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because file data in line 10

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is actually referring to this variable file data

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because they are in the same block of code.

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File data defined here,

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this constant file data which we are assigning in line 6,

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is not the file data we are referring to here,

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because file data here in line 6 is in a different block,

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in that try block.

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Now that's why we get undefined here

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when we try to refer to file data.

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It's simply referring to a different file data

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which hasn't received a value.

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But why aren't we getting an error

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if we try to redefine a variable or constant

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that already exists?

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Typically that's not possible.

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For example, I wouldn't be allowed

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to add a constant file data in the very next line

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after defining that variable with the same name.

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Well that's again related to scoping.

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If you do create variables or constants

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with clashing names in different scopes, that's allowed.

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Here in try, in a scope that's part of

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this overall read file function scope,

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I can create a new constant,

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and if that constant has the name of a variable

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or constant of the outer scope, that is allowed,

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and we simply shadow that outer variable or constant.

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That means we can use the outer variable or constant

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if you want to, like in this case,

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when I assign a value to it.

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But we can also create a different constant

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or variable of the same name,

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which will then have a higher priority

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inside of that inner block.

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So then we simply have a second constant,

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which is totally unrelated to this other variable

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or constant with the same name.

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This is something you can do.

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It's also not uncommon to do that,

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though if you find something like that to be confusing,

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you can of course always go for different names

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to make it clearer,

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which variable or constant belongs to which scope.

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Now here I will remove const again,

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but being aware of this shadowing concept,

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and that you can have clashing names

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is something that you should keep in mind.

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But that's already all you need to know about this feature.