What is a 'Closure'?

Question

I asked a question about Currying and closures were mentioned. What is a closure? How does it relate to currying?

Answer 1

A closure is a stateful function that is returned by another function. It acts as a container to remember variables and parameters from its parent scope even if the parent function has finished executing. Consider this simple example.

  function sayHello() {
  const greeting = "Hello World";

  return function() { // anonymous function/nameless function
    console.log(greeting)
  }
}


const hello = sayHello(); // hello holds the returned function
hello(); // -> Hello World

Look! we have a function that returns a function! The returned function gets saved to a variable and invoked the line below.