Closure in JavaScript

In JavaScript, a closure is a combination of a function and the lexical environment within which that function was declared.

This environment consists of variables and values available at the time the closure was created. Closures allow a function to access and remember the scope in which it was created, even after that scope has finished executing.

Here's a simple example to illustrate closures:

function outerFunction() {
  // Outer function has a variable 'outerVar'
  let outerVar = 'I am from outerFunction';

  function innerFunction() {
    // Inner function can access 'outerVar'
    console.log(outerVar);
  }

  return innerFunction; // Return the inner function, creating a closure
}

const closureExample = outerFunction();

// Invoke the closure, which remembers the 'outerVar' from its lexical scope
closureExample(); // Output: 'I am from outerFunction'
        

In this example, innerFunction is defined inside outerFunction, and it has access to the outerVar variable. When outerFunction is invoked, it returns innerFunction, creating a closure. The closure retains access to the outerVar variable even though outerFunction has finished executing.

Closures are powerful because they provide a way to create private variables, maintain state, and encapsulate functionality. They are commonly used in scenarios like creating factory functions, managing private data, and implementing callback functions.