What is a Closure in JavaScript?

A closure in JavaScript is a function that remembers the variables from its outer lexical scope even after the outer function has finished executing.

In other words:

• A closure gives a function access to an outer function’s variables, even after the outer function has executed.
• The function “closes over” the variables from its outer scope.

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Key Characteristics of Closures:

• Closures remember the environment in which they were created.
• Closures can access variables from their outer scope even after the outer function is done executing.
• Closures are created every time a function is created inside another function.

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Example 1: Basic Closure Example

javascriptCopyEditfunction outer() {
    let count = 0; // Variable in outer scope

    function inner() {
        count++; // Accessing outer scope variable
        console.log(count);
    }

    return inner;
}

const counter = outer(); // outer() is executed, but its scope is preserved
counter(); // 1
counter(); // 2
counter(); // 3

Explanation:

• outer() creates the variable count.
• inner() is defined inside outer() and has access to count.
• Even though outer() is done executing, the counter function still remembers count because of closure.
• Each time counter() is called, it remembers the previous state of count.

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Example 2: Closure with Parameters

javascriptCopyEditfunction multiplier(factor) {
    return function (number) {
        return number * factor;
    };
}

const double = multiplier(2);
const triple = multiplier(3);

console.log(double(5)); // 10
console.log(triple(5)); // 15

Explanation:

• multiplier() returns a function that multiplies by factor.
• The returned function remembers factor due to closure, even after multiplier() has finished execution.

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Why Are Closures Useful?

• Data privacy: Variables in closures are not accessible from outside.
• Stateful functions: Functions can remember state between calls.
• Callbacks and Event Handlers: Closures are common in asynchronous programming and event handling.

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Common Use Cases of Closures:

Use Case	Example
Data hiding/encapsulation	Counter, private variables
Factory functions	Generating customized functions
Callbacks and async functions	Event listeners, setTimeout
Function currying	Partial application of functions

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Key Takeaways:

• A closure is a function + the environment in which it was created.
• Closures allow functions to remember and access their outer scope variables even after the outer function has completed.
• Used heavily in JavaScript patterns like module pattern, callbacks, and functional programming.