What Are Closures in JavaScript?

Closures are a fundamental concept in JavaScript that can seem tricky but are incredibly powerful. Here's a quick guide to understanding them.

What is a Closure?

A closure is created when a function "remembers" the variables from its lexical scope, even after that scope has exited.

Example:

function outerFunction(outerVariable) {  
  return function innerFunction(innerVariable) {  
    console.log(`Outer: ${outerVariable}, Inner: ${innerVariable}`);  
  };  
}  

const myClosure = outerFunction("Hello");  
myClosure("World");  
// Output: Outer: Hello, Inner: World

Here, innerFunction has access to outerVariable from outerFunction's scope, even after outerFunction has finished executing.

Why Are Closures Useful?

Data Privacy: Create private variables.

function counter() {  
  let count = 0;  
  return function () {  
    count++;  
    return count;  
  };  
}  
const myCounter = counter();  
console.log(myCounter()); // 1
console.log(myCounter()); // 2  

Partial Application: Pre-fill function arguments.

function multiplyBy(multiplier) {  
  return function (number) {  
    return number * multiplier;  
  };  
}  
const double = multiplyBy(2);  
console.log(double(5)); // 10  

Conclusion

Closures let functions remember their environment. They’re great for private variables, partial application, and many other advanced patterns in JavaScript.