Closures in javascript

Introduction

JavaScript closures are essential but sometimes confusing concepts in the JavaScript language. Understanding closures can help you write more efficient code and become a better JavaScript developer.

Closures are created when a function is defined inside another function, and the inner function refers to the variables declared in the outer function's scope chain. In other words, a closure is created when an inner function has access to the variables of an outer function.

Note

Whenever a function is created in JavaScript, it's automatically a closure.

In this blog, we'll explore what closures are, how they work, and provide code examples to help you better understand them.

Concepts of closures

To understand closures conceptually, we need to understand two main concepts: variable scope and the execution stack.

Variable scope

Variable scope in JavaScript determines which variables are accessible at any given time or from any given place.

When a variable is declared inside a function, it is said to be in the function's scope, which means other functions outside of that scope cannot access the variable.

Here is an example:

The code logs "Chloe" because the inner function innerScope has access to the variable name declared in the outer function scopeExample.

Execution stack

The execution stack is a stack data structure that JavaScript uses to organize and keep track of the order in which functions are called.

Each time a function is called, it is added to the top of the stack. When the function completes execution, it is removed from the stack.

Here's an example to illustrate how functions are added and removed from the execution stack:

The output is:

1. Inside first function
2. Inside second function

This happens because firstFunction is called first, and then it calls secondFunction. When secondFunction completes execution, it is removed from the stack, and then firstFunction completes execution and is removed from the stack.

Now that we have covered variable scope and the execution stack, let's dive into closures.

How closures work

Closures are created when an inner function has access to the variables of an outer function even after the outer function has completed execution and been removed from the stack.

Consider the following code:

The outerFunction function returns a reference to the innerFunction. We store this reference in a variable myName.

myName now has access to the name variable declared in outerFunction even though outerFunction has completed execution and has been removed from the stack.

This is because JavaScript maintains a reference to the name variable in the closure of innerFunction. A closure is created when innerFunction is defined inside outerFunction, and innerFunction has access to variables in outerFunction.

Examples of closures

Here are a few examples of how closures are used in real-world JavaScript code.

Example 1: Private variables

Closures are often used to create private variables in JavaScript.

In this example, login and logout functions have access to the variables username and password, but these variables cannot be accessed from outside the user function.

This means that username and password are private variables. login and logout can use them but cannot be accessed from outside the user function.

Creating event listener

Closures are frequently used in event listeners.

In this example, the init function has access to the button variable. The addEventListener function creates a closure that has access to the button variable, even after the init function has completed execution and been removed from the stack.

When the button is clicked, the closure executes and displays an alert message, demonstrating the concept of closures in event listeners.

Conclusion

Closures are an essential part of the JavaScript language that every JavaScript developer should understand. A closure is created when an inner function has access to variables in its outer function even after the outer function has completed execution and been removed from the stack.

Closures are often used to create private variables, event listeners, and much more, making them one of the most fundamental concepts in JavaScript.

I hope this blog helped you understand closures better. Thank you for reading!