JS closure and how it behaves

mrchow19910319 · September 23, 2017

I am reading through YDKJS scope and closure and here are something I wanna ask you guys. In the book it stated these:

function foo() {
	var a = 2;

	function bar() {
		console.log( a );
	}

	return bar;
}

var baz = foo();

baz(); // 2 -- Whoa, closure was just observed, man.

Quote

The function bar() has lexical scope access to the inner scope of foo(). But then, we take bar(), the function itself, and pass it as a value. In this case, we return the function object itself that bar references.

After we execute foo(), we assign the value it returned (our inner bar() function) to a variable called baz, and then we actually invoke baz(), which of course is invoking our inner function bar(), just by a different identifier reference.

bar() is executed, for sure. But in this case, it's executed outside of its declared lexical scope.

After foo() executed, normally we would expect that the entirety of the inner scope of foo() would go away, because we know that the Engine employs a Garbage Collector that comes along and frees up memory once it's no longer in use. Since it would appear that the contents of foo() are no longer in use, it would seem natural that they should be considered gone.

But the "magic" of closures does not let this happen. That inner scope is in fact still "in use", and thus does not go away. Who's using it? The function bar() itself.

By virtue of where it was declared, bar() has a lexical scope closure over that inner scope of foo(), which keeps that scope alive for bar() to reference at any later time.

bar() still has a reference to that scope, and that reference is called closure.

So, a few microseconds later, when the variable baz is invoked (invoking the inner function we initially labeled bar), it duly has access to author-time lexical scope, so it can access the variable a just as we'd expect.

The function is being invoked well outside of its author-time lexical scope. Closure lets the function continue to access the lexical scope it was defined in at author-time.