What are closures and how are they useful?

In JavaScript, a closure is a function that has access to the variables and functions defined in its outer scope, even after the outer function has returned.

Here is an example of a closure in JavaScript:

function outerFunction() {
  let outerVariable = 'I am the outer variable';

  function innerFunction() {
    console.log(outerVariable);
  }

  return innerFunction;
}

let myClosure = outerFunction();
myClosure();  // logs "I am the outer variable"