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Understanding Closure in JavaScript and Its Importance

Posted on:September 1, 2023 at 12:00 AM (3 min read)

Introduction

Welcome, fellow coders! Today, we’re going to delve deep into one of the most important and powerful features of JavaScript - Closures. This concept is often puzzling for beginners, but once you get a grasp of it, you’ll find it immensely useful in your coding journey. Let’s break it down in an easy-to-understand manner.

What is a closure?

In simple terms, a closure is a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In JavaScript, a closure gives you access to an outer function’s scope from an inner function.

To understand it better, we need to comprehend two fundamental concepts in JavaScript: scope and lexical scope.

How does a closure work?

Let’s see this in action with a basic example:

function outerFunction(outerVariable) {
  return function innerFunction(innerVariable) {
    console.log("outerVariable:", outerVariable);
    console.log("innerVariable:", innerVariable);
  };
}
const newFunction = outerFunction("outside");
newFunction("inside"); // Logs: outerVariable: outside, innerVariable: inside

In this example, innerFunction has access to variables in its own scope (innerVariable), the outer function’s scope (outerVariable), and the global scope. Even after outerFunction has finished execution, newFunction still has access to outerVariable. This is the core of what a closure is.

Why are closures important?

  1. Data privacy: Closures are commonly used to give objects data privacy. Data privacy is an essential aspect of coding where you can shield certain data from the rest of the program.

  2. Function factories: A closure can be used to create multiple functions using the same function body but holding different state information. This is fundamental in creating functions in a loop.

  3. Event handlers and callbacks: Closures are widely used in event handlers and callback functions where you might want a function to have access to variables set at the time of creation of the function.

  4. Function currying: Function currying in JavaScript is a technique of evaluating a function with multiple arguments, into a sequence of functions with single argument. In essence, when a function, instead of taking all arguments at one time, takes the first one and return a new function that takes the second one and returns a new function which takes the third one, and so forth, until all arguments have been fulfilled.

Closures in practice

Here’s a practical example of how closures can be used to create private variables:

function Counter() {
  let count = 0;

  this.increaseCount = function () {
    count++;
    console.log(count);
  };
}

const counter = new Counter();
counter.increaseCount(); // Logs: 1
counter.increaseCount(); // Logs: 2

In this example, count is private - it can’t be accessed directly from outside the Counter function or overridden. We can only increment it using the increaseCount method. This is achieved using a closure, which allows the increaseCount function to access count from its parent scope.

Conclusion

Closures in JavaScript might seem tricky at first, but they are an essential concept that brings immense power and flexibility to the language. Closures allow functions to maintain state and privacy, which makes your code more robust and secure.

Remember, understanding closure requires a solid understanding of scopes and how functions are executed in JavaScript. The more you code, the more you’ll understand and appreciate this fascinating concept. Happy coding!