Okay. So here I am in Sublime code editor. And I'm located in the script.js, which is located in the js folder of the lecture 42 folder. And again, I opened up Sublime the same way I've been opening it up for the last couple of lectures. Let's go ahead and close the file browser. And let's take a look as to what we have over here. So here we defined the variable X and then print it out straight to the console. Let's go ahead and do that by saving the file and as we can see, the value of the variable X is undefined. Well the reason it's undefined is because it's never been set to anything. So if the next line I say X=5 and save it again, the value would be five. Okay, so let's go erase that for a second. And let's go and uncomment this line of code right here. So this line of code directly tests whether or not X is actually equal to undefined. And if it is, it will go ahead and print out to the console X is undefined. So let's save this and see what our console says. And sure enough, X is undefined. This kind of demonstrates to you that this is a reserved keyword, that you can actually test against to see if it's undefined or not. You have to be a little bit careful, because JavaScript has this concept of type coercion which we haven't covered yet. But in general, this is something you could definitely test. Now let's see what happens if we uncomment this. And what this does is actually sets the value to 5, so now X is explicitly defined. And if we test it against the undefined keyword, it should jump out of the if statement. And we haven't covered if statements, but if statements are pretty generic across many languages so I'm sure you understand this code here. So if x is equal to undefined, then we'll say x is undefined, otherwise else we'll just say x has been defined. So what is it going to do? It first should tell us x is undefined and then it should jump and tell us x has been defined. Let's go ahead and save that.
So we get our undefined, x is now undefined and x has been defined. Okay, so, all this is well and good. Let's go ahead and comment this out for a minute. And, let's do this, let's comment out the actual definition of x. If we comment out the actual definition of x and let's try to print out x. And, if we go ahead and save that we're going to get an error. And the error message is a little bit confusing. It says, "Uncaught ReferenceError: x is not defined". Well, what, is this a joke? We just said that x is undefined, so is undefined not the same as not defined? And that is correct. Undefined in JavaScript has a specific meaning and the meaning of it is, is that it has been declared, or defined, so to speak. But it has not been set to any value. So in this case the variable has been declared and a memory space has been allocated for it, but no value has been placed in that memory space. That's the state that assigns this variable to be a type of undefined. Now if I save that, now it will tell me that the variable is actually undefined.
So to summarize, JavaScript defines seven built-in types, one object and six primitives. Now object type is a collection of name value pairs. It is nothing more than that in JavaScript. A primitive type can only contain a single, immutable value. And remember thar immutable means that once you set it, it cannot be changed. You could create new values based on this values, but you cannot change the original value anymore. Now, there's a particular interesting type called undefined among the primitives, and undefined means variable memory has been allocated, but not value has ever been explicitly set yet. And it's important to distinguish between that and a variable that has never been defined, in which case if you try to use it, you're going to get a reference error. And it will be a real error that will pop out in your browser console.
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