Learn Closures In 7 Minutes

Web Dev Simplified

3 Dec 201906:55

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

00:00

📘 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.

05:00

🔄 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

Closures are a fundamental concept in JavaScript that allow a function to have access to its own scope, the outer function's scope, and the global scope, even after the outer function has completed execution. In the video, closures are introduced as a simple yet powerful feature that enables functions to 'remember' the environment in which they were created. An example given is a function that prints a name, which continues to have access to a variable defined in the outer scope, demonstrating how closures work in practice.

💡Scope

Scope refers to the accessibility of variables within different parts of a program. In JavaScript, every function creates a new scope. The video explains that closures allow functions to access variables from their outer scope, which is crucial for understanding how closures function. The script mentions 'outer scope' and 'entire JavaScript file' as different levels of scope that closures can access.

💡Global Variable

A global variable is a variable that is defined outside of all functions and is accessible from any scope within the JavaScript code. In the context of the video, the global variable 'my name' is used to demonstrate how closures enable a function to access and modify a global variable, even after the function has been defined.

💡Function

A function is a block of code designed to perform a specific task, which can be called by using its name followed by parentheses. The video script uses functions to illustrate closures, showing that functions can access variables from their outer scope. The 'print name' function in the script is an example of how a function can utilize a closure to access an outer variable.

💡Inner Function

An inner function is a function defined within another function. The video explains that inner functions have access to the variables of their outer function due to closures. This is demonstrated with an 'outer function' that returns an 'inner function', which can access the 'outer variable' even after the outer function has finished executing.

💡Outer Function

An outer function is a function that contains another function within it. In the video, the concept of an outer function is used to explain how closures allow the inner function to access its variables. The 'outer function' in the script sets an 'outer variable', which is then accessible by the returned 'inner function'.

💡Variable

A variable is a storage location paired with an associated symbolic name, containing some known or unknown quantity of information referred to as a value. The video script discusses how variables, such as 'my name' and 'outer variable', are used in closures to demonstrate the concept of functions accessing and modifying variables from their outer scope.

💡Live Value

The live value of a variable refers to its most current value at any given moment in the program's execution. The video explains that closures capture the live value of a variable at the time the inner function is called, not just the value at the time the function was defined. This is shown when 'my name' is changed to 'Joey' and the function still prints the updated value.

💡Axios/Fetch

Axios and Fetch are libraries and a web API, respectively, used to make HTTP requests from a web page. The video script mentions these as examples of where closures might be used in practice, such as within a function that handles a fetch request, where an inner function can access variables set in the outer function, like the URL being fetched.

💡Promise

A promise in JavaScript is an object representing the eventual completion or failure of an asynchronous operation. In the video, a promise is used in the context of a fetch request, where the video script explains that an inner function can access the outer scope variables even after the outer function has completed, which is a common use case for closures in handling asynchronous operations.

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.