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

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 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 created a variable a is equals to 7 right so even before this line of code is executed right javascript has tried to allocate memory to this a right memory space to a even before this line of code is run okay let me show you 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 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 console if you try to log what a is see i have hold the program i have held the program like i 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 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 lock this so 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 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 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 7 to a i am 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 with javascript 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 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 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 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 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 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 equal to undefined so equal equal equal to you might not be aware of the 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 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 um over here let us just log it a is undefined okay and if we try to do something like 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 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 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 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 it it is like 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 just like other languages just like c or c plus plus if you assign a variable 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 var a for the time being it can like just hold undefined okay if i do a console 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 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 is a perfectly fine javascript code okay and if i log this a once again okay so see what the output will be undefined so until unless any value is specified there is a placeholder undefined when you have put in 10 so it is 10 now and when you put in string hello world it is hello world now ok so javascript is very flexible in this case that is why javascript is known as loosely type language loosely typed language are also known as weekly typed language weekly type language so it does not mean that it is weak in any sense it does not have any deficiency 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 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 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 weekly type language okay so let me 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 possible to assign and define 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 first of all what is undefined so undefined is like a placeholder which is kept inside the variables 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 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 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 uh know okay that whether that variable was assigned 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 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 javascript [Music] you

Channel: Akshay Saini

Views: 121,862

Rating: 4.986691 out of 5

Keywords: Javascript course, Namaste javascript, akshay saini, akshay saini javascript, javascript tutorials, best javascript course, javascript video series, javascript fundamentals, namaste js, javascript core concepts, advanced javascript, undefined, undefined in js, undefined in javascript, undefined vs not defined, not defined in js, what is undefined, javascript typeof undefined, javascript check if undefined, null and undefined in javascript, check for undefined in javascript

Id: B7iF6G3EyIk

Length: 11min 1sec (661 seconds)

Published: Fri Oct 23 2020