If you want to truly understand the working of JavaScript, Closure is one of the essential topics you must understand thoroughly.

I myself struggled to understand this topic in the beginning after researching I came across a Video course by "Will Sentance" with the name JavaScript: The Hard Parts, v2.

Which elaborates on this topic in a very easy-to-understand way.

Why closure is vital to understand?

• It is the basis for understanding other essential concepts like iterators, generator functions, memoization, etc.

• It is also one of the most asked questions in interviews.

What is the closure?

The closure is a functionality of JS by which functions remember the data from their previous running.

Normally, when we start running a function we create a live store of data related to that function called Memory or environment variable.

When we return from that function all this memory is deleted except the returned value.

But in the case of closure, we return a function and even after returning it remembers or has a hold of the data or memory from the previous running.

It starts with returning a function from a function

As in JavaScript, the functions are first call members which means they can be passed to or returned from other functions like any other data variable. The function can be returned from a function in JavaScript.

function outerFunction() {
    return function innerFunction(num) {
       return num*2;
    }
}

const multiplyByTwo = outerFunction();
const result = multiplyByTwo(2);

Now, when the function outerFunction() runs and encounters the return statement then the whole function definition of innerFunction() gets returned and outerFunction's execution context gets deleted.

This definition is stored in the const multiplebyTwo.

So, now when we run multiplebyTwo we will be running a function which was born as innerFunction in the execution context of outerFunction.

This returned function, multiplyByTwois an example of a closure. It retains access to the variables and scope of its outer function, outerFunctioneven after the outer function has finished executing and its execution context has been deleted.

The closure allows multiplyByTwo us to "remember" the value of the num parameter and innerFunction itself. This means that when we invoke, it can still access and utilize the num parameter with the value of 2, as well as any other variables or functions defined in the outer function.

The concept of closures becomes particularly powerful when we consider that the outer function's execution context is not limited to just variable declarations. It can include other nested functions, objects, and more. The closure captures the entire state of its outer function, providing a powerful mechanism for maintaining data privacy and encapsulation in JavaScript.

Closures are commonly used for various purposes in JavaScript. Here are a few examples:

1. Private Variables: By using closures, we can create functions with private variables that are inaccessible from the outside world. This technique is often referred to as the "module pattern" and is widely used in JavaScript code organization.

2. Function Factories: Closures can be used to create function factories, where a higher-order function generates and returns specialized functions based on certain parameters or configurations.

3. Asynchronous Operations: Closures are commonly employed when dealing with asynchronous operations such as event listeners, timeouts, or AJAX requests. They allow the callbacks to access the necessary variables and context even after the initial function has been completed.

4. Memoization: Closures can be used for implementing memoization, a technique that stores the results of expensive function calls and returns the cached result when the same inputs are provided again.

Understanding closures is crucial not only for solving complex programming challenges but also for comprehending more advanced JavaScript concepts and patterns. It allows you to write more efficient, modular, and maintainable code.

In conclusion, closures in JavaScript provide a way to retain access to variables and functions from an outer function's scope even after its execution has finished. They enable powerful techniques such as private variables, function factories, handling asynchronous operations, and memoization. By grasping the concept of closures, you'll unlock a deeper understanding of JavaScript and be better equipped to tackle various programming tasks.