AI solution for Interview Question on BFE.dev
4. what is Closure and how does it work?

In simple terms, a closure is a function defined inside another function that retains access to the variables and parameters of its outer function, even after the outer function has returned. The closure function has access to these variables because they are still in memory.

When a function returns, its local variables and parameters are usually removed from memory. But, if an inner function has a reference to those variables, they are not garbage-collected, and the inner function still has access to their values.

This is also useful for creating private variables, as the inner function can access the outer function's variables, but they are not directly accessible from outside the closure.

Here is an example of a closure:

function greeting(name) {
  var greetingMessage = "Hello, ";

  function displayGreeting() {
    console.log(greetingMessage + name);
  }
  
  return displayGreeting;
}

var sayHello = greeting("John");

sayHello(); // outputs "Hello, John"

In this example, the inner function displayGreeting is returned from the outer function greeting, and it retains access to the greetingMessage variable and the name parameter. The reference to the displayGreeting function is stored in the sayHello variable, which can be called to output the greeting.