Learn Closures In 7 Minutes
Summary
TLDRIn this video, Kyle from Web Dev Simplified explains the concept of closures in JavaScript. He begins with a simple example, demonstrating how variables outside a function can be accessed within it, forming a closure. Kyle illustrates this with an example where a function accesses a global variable, showing how the variable's value updates dynamically. He then delves into more common use cases, such as functions within functions and how closures retain access to variables even after the outer function has executed. The video concludes with practical applications of closures using fetch and promises in JavaScript.
Takeaways
- 😀 Closures in JavaScript allow functions to access variables from their outer scope even after the outer function has finished executing.
- 🎯 The video introduces closures with a simple example involving a 'printName' function and a global variable 'myName'.
- 🌟 The script demonstrates that closures are not just about accessing the value at the time of function creation, but the current live value of the variable.
- 🔄 It shows that changing the value of a variable after a function has been defined can still affect the output when the function is called later.
- 📚 The video explains that every scope in JavaScript, including the entire file and any functions within it, has access to the outer scope.
- 🏢 The concept of closures is further illustrated with an 'outer function' returning an 'inner function', highlighting how the inner function retains access to the outer variable.
- 🔑 The script clarifies that even when the outer function's execution is complete, the inner function maintains access to its variables due to closures.
- 🛠️ Common use cases of closures are shown, such as using functions within functions to maintain access to scoped variables, which is useful in asynchronous operations like fetching data.
- 🌐 An example with 'fetch' is given to explain how closures can be used to access variables defined in an outer function within a callback function of a promise.
- 👨🏫 The video aims to simplify the concept of closures and encourage viewers to explore more through subscribing to the channel for related content.
- 📈 The presenter emphasizes the practical applications of closures, particularly in scenarios involving asynchronous operations and callbacks.
Q & A
What is the main topic of the video?
-The main topic of the video is explaining the concept of closures in JavaScript.
What is the presenter's name and the purpose of his channel?
-The presenter's name is Kyle, and the purpose of his channel is to simplify web development concepts for the audience.
Why is the simple JavaScript code with the variable 'myName' and the 'printName' function considered a closure?
-The simple JavaScript code is considered a closure because the 'printName' function has access to the 'myName' variable, which is in an outer scope, demonstrating the ability to access external variables within a function.
How does the video demonstrate that closures update with the current live value of a variable?
-The video demonstrates this by changing the value of 'myName' from 'Kyle' to 'Joey' and showing that the 'printName' function prints the updated value, indicating that closures capture the current state of the variable at the time of function execution.
What is the difference between a global variable and a variable in a closure?
-A global variable is accessible throughout the entire script, while a variable in a closure is accessible only within the function that creates the closure, even after the outer function has finished executing.
Can you give an example of a more common use case of closures mentioned in the video?
-A more common use case of closures mentioned in the video is when a function (outer function) returns another function (inner function), and the inner function has access to the outer function's variables.
How does the video explain the scope concept in relation to closures?
-The video explains that in JavaScript, every scope, such as the entire file or a function, has access to everything in its outer scope. This is how closures work, allowing functions to access variables from their parent scopes.
What happens to the outer variable after the outer function has finished executing in the context of closures?
-In the context of closures, even after the outer function has finished executing, the inner function retains access to the outer variable because it 'remembers' the outer variable's value at the time of its creation.
Why are closures particularly useful in asynchronous operations like fetching data with Axios or fetch?
-Closures are particularly useful in asynchronous operations because they allow the inner function, which is called after the outer function has completed, to still have access to the outer function's scope, including variables that are needed to process the data once the promise resolves.
Can you provide an example of how closures are used with asynchronous functions like fetch?
-The video provides an example where an outer function is defined to fetch a URL, and the fetch function's '.then()' method contains an inner function that has access to the outer function's variables, such as the URL, even after the outer function has finished executing.
What is the key takeaway from the video about closures?
-The key takeaway from the video is that closures in JavaScript allow an inner function to have access to the variables and scope of an outer function, even after the outer function has finished executing.
Outlines
📘 Introduction to Closures in JavaScript
In this introductory segment, Kyle from Web Dev Simplified explains the concept of closures in JavaScript. He begins by demonstrating a simple JavaScript code example that uses a global variable and a function to print a name, which is a basic form of a closure. Kyle clarifies that closures allow functions to access variables from their outer scope, even after the outer function has finished executing. He also touches on the dynamic nature of closures, showing how they update to reflect the most recent value of a variable. The explanation is aimed at simplifying the understanding of closures, which are often misunderstood due to their more complex use cases.
🔄 Practical Use of Closures with Functions and Promises
This paragraph delves into the practical applications of closures, focusing on their use within functions that return other functions. Kyle illustrates this with an example of an 'outer function' that returns an 'inner function' and demonstrates how the inner function maintains access to the outer function's variables, a key feature of closures. He further explains how closures work with asynchronous operations, such as those involving promises with Axios or fetch, where an inner function can access variables from the outer function's scope even after the outer function has completed. The summary emphasizes the importance of closures in creating encapsulated and reusable code, particularly in scenarios involving asynchronous operations.
Mindmap
Keywords
💡Closures
💡Scope
💡Global Variable
💡Function
💡Inner Function
💡Outer Function
💡Variable
💡Live Value
💡Axios/Fetch
💡Promise
Highlights
Closures in JavaScript allow functions to access variables from their outer scope even after the outer function has finished executing.
The concept of closures is often misunderstood but is actually quite simple to grasp once explained.
A basic example of a closure is a function that prints a name, which in this case is 'Kyle', using a variable defined outside the function.
Closures demonstrate that JavaScript functions have access to the outer scope, including global variables.
Changing the value of an outer variable after a function has been defined affects the function's output, showing it captures the most recent value.
Closures are not just limited to accessing variables; they also allow functions to access other functions defined in their outer scope.
An 'outer function' returning an 'inner function' is a common use case for closures, where the inner function retains access to the outer variable.
The inner function 'saves' the outer variable, ensuring access to its value even after the outer function has completed execution.
Closures are useful for creating private variables within functions, as they encapsulate the outer scope's variables.
Functions defined within other functions have access to all parent scopes, including variables and other functions.
The video provides an example of using closures with asynchronous operations like fetching data from a URL.
Closures are particularly useful in scenarios involving promises and callbacks, where access to outer scope variables is necessary.
Arrow functions, despite their different syntax, also exhibit closure behavior, maintaining access to the outer scope.
The video simplifies the understanding of closures by demonstrating them with practical examples and clear explanations.
Closures enable the creation of functions with 'memory', remembering the state of variables from their outer scope.
The video concludes by emphasizing that closures are about functions having access to the scope of their outer function, regardless of when they are called.
Transcripts
hello everyone in today's video I'm
gonna be explaining to you exactly what
closures are and you'll be surprised how
simple they are to understand once you
wrap your head around the idea so let's
get started now welcome back to web dev
simplified my name is Kyle and my job is
to simplify the web for you so if that
sounds interesting make sure you
subscribe to the channel for more videos
just like this one and now to get
started I just have really simple
JavaScript on the left side here and
then the output showing up on the
right-hand side and this is very simple
code we have a variable called my name a
function called print name which prints
out our name whenever we call it and
then we're calling that function and as
you can see we're getting Kyle printed
out over here on the side and what you
probably don't realize is this is using
closures this entire thing is one giant
closure and that's because most of the
time when you run code in other
languages for example you cannot access
variables outside of a function inside
of that function that is just not always
possible in other languages but it is
possible in JavaScript and it is what we
call a closure so this variable that is
external to this function is actually
available internally inside this
function this is a global variable
essentially and it's available inside
this function and the way closures work
in JavaScript is that every scope so for
example we have the entire JavaScript
file that is one scope and then our
function is another scope so every scope
has access to everything outside of its
scope so our function has an outer scope
of our entire file so it has access to
everything inside the outer file and one
thing that's really interesting if we
change this here to a variable that can
be renamed so we have our name as Kyle
up here and we change our name down here
we'll just say that the new name is
going to be Joey and we save that you
can see this now prints out Joey and so
it's not just taking the name that is
defined when the function is defined
it's actually taking the current live
value of that name it goes for example
if we have the name here and then we
come down here change
- something else will change it decayed
and then we print out the name one more
time you can see that it says Joey and
then Kate it's constantly going with
whatever the most recent value of that
variable is and now this right here is
not the most useful way that you can use
closures I mean it obviously is a way
you can use them but when most people
think of closers they think of functions
inside of other functions so let me jump
to another example now so here is the
more common use case of closures where
most people talk about closures and as
you can see we have a function here
called outer function and that function
is returning another function inside of
it called inner function and as you can
see down here we're calling outer
function with the variable outside so
now we are getting a new function and
then lastly we're calling that new
function with the variable inside so
what's happening is that when we first
call this outer function we have this
outer variable which we set to outside
which gets set and then we have this
function that gets returned here and the
reason we're able to access this outer
variable inside of this function is
because of closures and now it's kind of
weird when you think about it at first
because this outer function runs and
this outer variable is only available
inside this function but this function
is done executing for example if we just
remove this code here you can see
nothing prints out and that's because we
call this outer function right here and
it executes all of the code and then
it's done executing and this outer
variable is no longer accessible for
example I can't log out the outer
variable if I try to it's just gonna
give me an error as you can see it
doesn't actually exist so how is the
inner function able to access this outer
variable even after it's done being
executed because the outer variable has
gone out of scope and that is where
closures come in so what happens is this
inner function is essentially saying
okay I'm inside this outer function it
has this outer variable so I'm going to
save this outer variable I'm going to
make sure I know what this outer
variable is at all times
and even if the function that defined
this variable is no longer
I'm executed anymore it exits out of the
function it's okay I'm still going to
keep track of the outer variable and
that's why you can see outer is being
printed out outside here and we had
in a variable also being printed out if
I for example passing an inner variable
you can see it gets printed out right
there and the most common use case for
this a lot of times is to have variables
that you pass into functions that can be
accessed inside of other functions we
can also create a variable inside of
here we're going to call this outer two
and we're going to set this equal to
high and we can also log that inside of
here so we can say console dot log I
would over to and you can see high is
being printed out again this is inside
the scope that is outside this function
so this function is contained inside
this function so everything in the outer
function is available inside the inner
function since in JavaScript anything on
the inside has access to the things on
the outside of its scope essentially it
has access to its parent scope its
parents parent scope and so on
everything that is outside of it and it
has access of course to the things
inside of it now this here is a fairly
contrived example so probably one of the
more common use cases where you're going
to see this is if you're using something
like Axios or fetch so if we come in
here and we have a function that's going
to do fetch so we want to fetch you know
for example some URL so we're gonna just
say you know URL whatever it is some
random URL that you're fetching and then
inside of here you're going to have an
actual promise that's being returned so
we can say dot been and this is going to
take a function and this function is a
function that's inside our outer
function and inside of here we would
have access to our outer variable so we
could say outer variable for example and
in most cases we'd probably pass in the
URL here and we pass it in here and
again we'd have access to that URL
inside this inner function and now
obviously this looks a bit different
this is an arrow function not a standard
function but either way it's still a
function defined inside of another
function and this inner function has
access to all of the scope of this outer
function so it has access to this URL
variable even though this fetch is only
going to call this dot vend function
long after outer function is done being
called really all you need to know about
closures is that when you have a
function defined inside of another
function like this that inner function
has access to the variables and scope of
the outer function E
if the outer function finishes executing
and those variables are no longer
accessible outside that function so if
you enjoyed that video make sure to
check out my other videos linked over
here and subscribe to the channel for
more videos just like this one thank you
very much for watching and have a good
day
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