Closure

what is closure?

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time. MDN ¹

In simple words we can access variables declared outside the function. Wow!

function foo() {
    let message = 'hello world';

    function speak() {
	console.log(message);
    }

    function setMessage(newMessage) {
	message = newMessage;
    }

    return [speak, setMessage];
}

const [greet, setMessage] = foo();

greet(); // hello world
setMessage("I'm Batman");
greet(); // I'm Batman

Here in the example, speak and setMessage was able to access message which was declared in foo’s scope. TaDa!! That’s it.