undefined vs not defined in JS 🤔 | Namaste JavaScript Ep. 6

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Transcript

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Undefined is a very special keyword in JavaScript and it is not there in other languages. It has a lot to do with how JavaScript code is executed.

So as we have studied in the previous videos also that JavaScript code is executed in a

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different way. It creates a global execution context and allocates memory to all the variables and functions even before a single line of code is executed.

Remember this. So undefined comes into picture here only.

Let us see with the code also. So suppose we

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created a variable a is equals to 7. Right?

So even before this line of code is executed, right? JavaScript is trying to allocate memory to this a.

Right? Memory space to a even before this line of code is run.

Okay? Let me show you

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with the help of debugger. So I am putting the debugger right before this line is executed.

Okay. So let us see an interesting thing.

So if we refresh the page, this A, this line hasn't been executed yet, right?

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But JavaScript has already allocated memory to this A. Okay?

So we have already reserved memory for A now. Okay?

So if we see here is the reserved memory. So right now A is undefined.

So at this point if you try to in the

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console, if you try to log what A is. See, I have hold the program.

I have held the program like I have put a debugger right so I have hold the program at that state itself. So if you try to log a right now it is undefined.

So why this happened? Okay so when it

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allocates memory to all the variables and functions to the variables it tries to put a placeholder right it is like a placeholder which is placed in the memory. Okay that special keyword is undefined.

So if we try to log this so

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you get undefined in it. So undefined is like taking memory it is it is very different than not defined.

Okay. So suppose if I try to access something else right.

So suppose if I try to do a console log of X over here. Okay.

So

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which we have not allocated memory to. Okay.

If we run this code if we run this code we have not allocated any memory to X. Right?

So what will happen if we try to find X? you won't find anything that is known as not defined but we have

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allocated memory to a if you try to console log a we can find that so even before I put this where a is equals to 7 right so suppose if I try to log a over here even before putting seven to a I am

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trying to log this a so what happens is what happens is let me just run it so In the console you will find that we have printed undefined. This a was printed undefined.

This is the value which JavaScript

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engine allocated to a while creating the memory. Okay.

But once this line has been executed after this line you will see that the value of a has changed to 7. Okay.

Let me just remove that x. If you try to log here see it was undefined

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before. This undefined was coming from the place just because JavaScript engine allocated undefined that special placeholder to a okay not like not defined not defined in something which

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has not been allocated memory okay so this is the difference between undefined and not defined okay we'll see something more so remember undefined is not equal to empty some people think that undefined means empty like it is not taking up any memory space or something.

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No, it is a special keyword. It takes up its own memory.

But you can assume it to be like a placeholder which is kept for the time being until the variable is assigned some other value. Okay.

Till that point, it will store this placeholder known as undefined. Okay.

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And one more thing I would show you right now is suppose if I do um suppose this is undefined, right? So if I never put any value inside a right like I just declared that okay where a and I never put any value to it.

So it will

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throughout our whole program have this value undefined that placeholder kept inside a. And to show you that it is something in JavaScript we can also do something like this.

Okay. Uh so suppose if I have my uh if a is equal equal

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equal to undefined. So equal equal equal to you might not be aware of this syntax but see it is just checking whether two things are equal or not and strictly checking we will talk about it later in the videos okay that will be interesting but for now just assume that it is checking two things are equal or not so

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if we try to see that a is equal equal to undefined okay that undefined is a thing or not so if we try to do a console log of over here let us just log it a is undefined Okay. And if we try to do something like

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else, we just console log a um string a is not undefined. Okay.

So if we try to check like this, what you will see? See a is undefined.

Okay. A is

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undefined. It is like it goes into this if it meets this condition.

It go inside this if block. And suppose if I put some value in a okay a is equals to 10 meanwhile so it will say that a is not

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undefined it has some value in it okay now the a is replaced that undefined is replaced by 10 interesting right now when we are talking about undefined let me tell you a little bit more about JavaScript and variables in it okay so you know JavaScript is a loosely typed

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language so loosely typed means that it does not attaches its variables to any specific data type. So suppose if I created a and put in string in it.

So later on in the program I can also put numbers in it. I can also put boolean in

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it. It is like uh very JavaScript is very flexible in this case.

Right? It is loosely typed.

That is known as loosely type language. If it was strict type then uh just like other languages just like C or C++ if you assign a variable

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that if it is a string a so it will only hold string it won't hold numbers or booleans or anything else but it is not the case in JavaScript you can put anything and everything inside this variable okay so if I do a

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for the time being it can like just hold undefined okay if I do a console So log over here it will just hold undefined. Okay.

And if I later on put a is equals to 10 here. So and try to log it again.

So it will it can hold numbers also. And

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it can also hold a string again. Okay.

So this is a perfectly valid JavaScript code. Okay.

Um I mistyped it. No issues.

So it it is a perfectly fine JavaScript code. Okay.

And if I log this a once

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again. Okay.

So see what the output will be undefined. So until unless any value is specified there is a placeholder undefined.

When you put in 10 so it is 10 now. And when you put in string hello world it is hello world now.

Okay. So

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JavaScript is very flexible in this case. That is why JavaScript is known as loosely type language.

Loosely type language are also known as weekly type language. Weekly typeyped language.

So it does not mean that it is weak in any sense. It does not have any deficiency

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or something. It is not weak at all.

In fact, I feel that the language is more stronger. Okay, because it is handling a lot of stuff behind the scenes.

Okay, you give it string, it handles automatically behind the scenes. It okay, it's a string.

You give it a

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number, it takes it and puts number in it and manages all these things behind it. So, it is not weak at all for sure.

In fact, I feel that it is more flexible and beautiful in this case. Okay.

So and it has to do a lot of things while

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coercion also. Okay.

When you change one type to another type, it is doing a lot of stuff. We might cover that in the later videos.

For now, just understand that JavaScript is a loosely typed language and some people also call it as a weakly type language. Okay.

So let me

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tell you an important thing. Never do this mistake.

Okay. Never do this mistake of doing something like this.

A is equal to undefined. What does that mean?

See, undefined is a value. It is a keyword in JavaScript.

And it is totally

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possible to assign undefined to any variable. But it is kind of a mistake and it is a bad thing to do in JavaScript.

Okay, you should not do it because see what first of all what is undefined? So undefined is like a placeholder which is kept inside the

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variables. Uh and it states that in the whole code that variable was not assigned anything right.

So it is meant for a specific purpose. So it is not generally a good way to put undefined like this.

Though it is a totally okay

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to do this. Okay.

If you just log it once again, if you log a once again, you will see that okay you you can put undefined once again. But it can lead to a lot of inconsistencies.

So it is not a good practice to do this. I won't say it's a mistake but it's not a good

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practice or a healthy practice to put undefined into any variable. So don't do this mistake.

Right? Let us just keep undefined for its own purpose.

Right? Undefined was meant to know okay that whether that variable was assigned

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anything or not. Okay?

So let it be used for that space only and for the JS community let us not ever put undefined into any variable. Okay.

So it's just a request. That's all for now.

In the next video we'll be covering the scope chain.

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It is a very beautiful topic I would say and it is very hot interview topic also. Okay.

In the interview a lot of people ask about what is a scope chain. So definitely watch that video.

So before moving on to that video, just like always give this video a thumbs up and thank you for watching Namaste

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JavaScript. [Music]