Course Overview

Overview of history of the JavaScript language

History of JavaScript

Overview of the relationship between JavaScript and Java

JavaScript vs Java

Overview of how JavaScript is evolved as a programming language

How is it evolved?

Overview of how we can track upcoming features in the JavaScript language

How to track upcoming features?

Overview of how the ECMAScript is versioned

How is ECMAScript versioned?

Explanation of whether JavaScript is interpreted or a compiled language

Is it interpreted or compiled?

Explanation of how JavaScript is executed

How is it executed?

Execution Context

Explanation of what is a callstack and how it is used in JavaScript

Callstack

Overview of memory management in JavaScript

Memory

What is JavaScript

What is hoisting?

Explanation of common misconception about hoisting in JavaScript

Misconception about hoisting

Hoisting

What is scope?

Explanation of global scope in JavaScript

Global scope

Explanation of function scope in JavaScript

Function scope

Block scope

Explanation of module scope in JavaScript

Module scope

Scope chain

Scope

What is coercion?

Diving into inner details of  coercion in JavaScript

Deeper look at coercion

Explanation of what abstract operations are and common types of abstract operations in JavaScript

Abstract operations

Explanation of how abstract or loose equality works in JavaScript

Abstract equality operator

Explanation of how the addition operator works in JavaScript

Addition operator

Explanation of how relational operators work in JavaScript

Relational operators

Coercion exercise for testing and practicing the knowledge learned in this module

Coercion exercise

Coercion

What is a closure?

Explanation of a common misconception about closures in JavaScript

Misconception about closures

Explanation of the problem of closures inside loops and how to fix it

Closures inside loops

Explanation of closures can be used to hide data

Data hiding using closures

Closures

What is prototypal inheritance?

Prototype chain

__proto__ property

Explanation of how we can create objects with custom prototypes in JavaScript

Custom prototypes

Explanation of how prototype chain is set with ES6 classes

ES6 classes and prototypes

Prototypes

Explanation of this keyword in JavaScript

What is this?

Explanation of how "this" works inside arrow functions

Arrow functions and "this"

Explanation of how value of "this" can be set explicitly

Explicitly binding "this"

globalThis

Summarizing "this"

'this' keyword

What is a symbol?

Explanation of why symbols were added in JavaScript

Why symbols?

Symbols and privacy

Explanation of global symbols in JavaScript

Global symbols

Explanation of what well-known symbols are in JavaScript

Well-known symbols

Symbol

Overview of asynchronous behavior of JavaScript

Overview

Overview of how callbacks help us write asynchronous code in JavaScript

Callbacks

Event loop

Promise

Explanation of promise chaining in JavaScript

Promise chaining

Explanation of some static methods of promises and their use cases

Use cases for static Promise methods

Explanation of async-await syntax in JavaScript

async await

Explanation of microtasks, how they are processed, and how they are linked to promises

Microtasks

Explanation of common anti-patterns to avoid when using promises in JavaScript

Promise anti-patterns

Asynchronous JavaScript

Explanation of what is an iterator in JavaScript

Iterators

Explanation of what is a generator in JavaScript

Generators

Explanation of how we can create asynchronous iterators in JavaScript

Asynchronous iterators

Explanation of how we can create asynchronous generators in JavaScript

Asynchronous generators

Iterators and Generators

Overview of what is debugging and different ways to debug JavaScript

Debugging

Overview of using the "debugger" statement to debug JavaScript

Debugger statement

Overview of using the breakpoints in browser developers' tools to debug JavaScript

Breakpoints in browser

Overview of using the VS Code debugger to debug JavaScript

VS Code debugger

Debugging JavaScript

Final thoughts and advice regarding how to continue the exciting journey of learning JavaScript

Conclusion

Wrap up

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Become a confident JavaScript pro by unraveling the most challenging aspects of the language. With easy-to-understand explanations and code examples, you’ll gain a deeper and solid understanding of JavaScript's most complex features.

Advanced JavaScript Unleashed is for beginner and intermediate developers alike, who want to elevate their JavaScript skills and become senior engineers by gaining a deeper understanding of the core topics of the language.

Advanced JavaScript Unleashed

Closure is one of the fundamental topics in JavaScript and can be tricky to understand for beginners. It is a powerful feature, and developing a good understanding of this topic is essential. In this module, we will take a deeper look at what closures are and how they work.

The closure is a combination of the following two things:

In other words, whenever we define a function in JavaScript, that function saves a reference to the environment in which it was created. This is what's referred to as a closure: a function along with a reference to the environment in which it is created.

Closures allow a nested function to access the declarations inside the containing function, even after the execution of the containing function has ended.

function outerFn() {
  const outerVar = 123;

  return function inner() {
    console.log(outerVar);
  };
}

const innerFn = outerFn();
innerFn();

 even after the "outerFn" execution is complete?

The above code works because JavaScript functions always create closures when they are created. In some programming languages, a function's locally defined variables only exist for the duration of that function's execution; when a function's execution ends, variables defined in its local scope are destroyed. But that's not the case in JavaScript, as is evident from the code example above. So, how do closures work?

To understand how closures work, we need to understand how JavaScript resolves the scope of any identifier.

Although we had a detailed discussion on scope in an earlier module dedicated to the topic of scope, let us once again briefly go over how the lexical scope is resolved. Consider the following code example:

let isReading = true;

function learnJavaScript() {
  console.log(isReading);
}

learnJavaScript();

 function is invoked to log the value of the 

 variable, the JavaScript first needs to identify where it is defined. The first place where JavaScript will search is the local scope of the 

 function because this is where a reference to the 

 variable is not defined in the local scope of the 

 function, JavaScript will search for this variable in the outer scope, which in this case is the global scope. JavaScript will find the declaration of the 

 variable in the global scope, so it will get its value and pass it to the 

 function so that it can be logged to the console.

Let us take a look at another code example that involves a nested function:

let isReading = true;

function learnJavaScript() {
  function stepsToLearnJavaScript() {
    console.log(isReading);
  }

  stepsToLearnJavaScript();
}

learnJavaScript();

In this code example, we have a nested function, 

, that logs the value of a variable defined in the global scope. If JavaScript hasn't already determined the scope of the 

 variable, how will it find its declaration? Recall the scope chain!

---
title: What is a closure?
description: Explanation of closure in JavaScript
isPublicLesson: false
useMdx: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/advanced-javascript-unleashed-app/-/tree/main/module_05/lesson_05.01
---

Closure is one of the fundamental topics in JavaScript and can be tricky to understand for beginners. It is a powerful feature, and developing a good understanding of this topic is essential. In this module, we will take a deeper look at what closures are and how they work.

## What is a closure?

The closure is a combination of the following two things:

-   A Function
-   A reference to the environment/scope in which that function is created

In other words, whenever we define a function in JavaScript, that function saves a reference to the environment in which it was created. This is what's referred to as a closure: a function along with a reference to the environment in which it is created.

Closures allow a nested function to access the declarations inside the containing function, even after the execution of the containing function has ended.

```js
function outerFn() {
  const outerVar = 123;

  return function inner() {
    console.log(outerVar);
  };
}

const innerFn = outerFn();
innerFn();
```

Here's a Replit of the code above:

<ReplitEmbed src="https://replit.com/@newlineauthors/Example1"/>

How does the `innerFn`, function returned by `outerFn`, have access to the `outerVar` variable declared in `outerFn` even after the "outerFn" execution is complete?

The above code works because JavaScript functions always create closures when they are created. In some programming languages, a function's locally defined variables only exist for the duration of that function's execution; when a function's execution ends, variables defined in its local scope are destroyed. But that's not the case in JavaScript, as is evident from the code example above. So, how do closures work?

## How do closures work?

To understand how closures work, we need to understand how JavaScript resolves the scope of any identifier.

Although we had a detailed discussion on scope in an earlier module dedicated to the topic of scope, let us once again briefly go over how the lexical scope is resolved. Consider the following code example:

```js
let isReading = true;

function learnJavaScript() {
  console.log(isReading);
}

learnJavaScript();
```

Here's a Replit of the code above:

<ReplitEmbed src="https://replit.com/@newlineauthors/Example2"/>

When the `learnJavaScript` function is invoked to log the value of the `isReading` variable, the JavaScript first needs to identify where it is defined. The first place where JavaScript will search is the local scope of the `learnJavaScript` function because this is where a reference to the `isReading` variable was found.

As the `isReading` variable is not defined in the local scope of the `learnJavaScript` function, JavaScript will search for this variable in the outer scope, which in this case is the global scope. JavaScript will find the declaration of the `isReading` variable in the global scope, so it will get its value and pass it to the `console.log` function so that it can be logged to the console.

Let us take a look at another code example that involves a nested function:

```js
let isReading = true;

function learnJavaScript() {
  function stepsToLearnJavaScript() {
    console.log(isReading);
  }

  stepsToLearnJavaScript();
}

learnJavaScript();
```

Here's a Replit of the code above:

<ReplitEmbed src="https://replit.com/@newlineauthors/Example3"/>

In this code example, we have a nested function, `stepsToLearnJavaScript`, that logs the value of a variable defined in the global scope. If JavaScript hasn't already determined the scope of the `isReading` variable, how will it find its declaration? Recall the scope chain!

JavaScript will need to traverse the scope chain to find the declaration of the `isReading` variable. As in the earlier example, the declaration is first searched in the current scope, which in this case is the local scope of the `stepsToLearnJavaScript` function. If the current scope doesn't contain the declaration JavaScript is looking for, the search is sequentially expanded to the outer scopes. In our example, the `stepsToLearnJavaScript` function doesn't contain the declaration of the `isReading` variable, so the search is moved to the outer scope, which in this case is the local scope of the `learnJavaScript` function. The declaration doesn't exist in this scope either; JavaScript traverses to the outer scope of the `learnJavaScript` function scope. The outer scope now is the global scope.