Function Declarations vs Expressions

function outer() {
  var x = 10;
  return function inner() {
    console.log(x);
  };
}
var closureFn = outer();
closureFn();

10
This code will output `10`. This example demonstrates a closure in JavaScript. A closure is formed when a function retains access to variables from its parent scope even after that parent function has finished executing. In this case, the `outer` function defines a variable `x` and returns the `inner` function, which references `x`. When `outer()` is called, it creates a variable `x` with the value `10` and returns the `inner` function, which we store in `closureFn`. Even though `outer` has finished executing at this point, the `inner` function (now assigned to `closureFn`) still maintains access to `x` through its scope chain. When `closureFn()` is later called, it can still access and log the value of `x` (which is 10), even though `x` was defined in `outer`, which is no longer in the call stack. This ability for functions to remember their lexical environment is a powerful feature of JavaScript that enables many programming patterns.
undefined
ReferenceError: x is not defined
null