When I wrote about functions in JavaScript I mentioned that functions are more than just a block of code:

Function is a standard data type in JavaScript, an object indeed; you can pass them around and copy them.

Let's take a look at this small sample of a function that creates and returns another function. The returned function accepts one string argument and returns another string repeating the argument a number of times.

function makeRepeater(times){
	return function(text){
		var message = '';
		for (var i=0; i < times; i++) {
			message += text + ' ';
		}
		return message;
	};
}

Let's now write some code that uses that function.

var threeTimes = makeRepeater(3);
var fourTimes = makeRepeater(4);
alert( threeTimes('hi') );
// => 'hi hi hi '
alert( fourTimes('hi') );
// => 'hi hi hi hi '

Nothing spectacular, right? But look closely. The function returned by makeRepeater contains a reference to times, which is a local variable of makeRepeater. When we call threeTimes or fourTimes the makeRepeater call has already returned and times should be out of scope. Or should it?

Extra life for your local scope

You may try to argue and say that the times inside threeTimes is not a reference to the times from makeRepeater, but just a copy of that value. Well, sadly I'll have to prove you wrong. Let's modify our code just a little.

var times;
function makeRepeater(){
	return function(text){
		var message = '';
		for (var i=0; i < times; i++) {
			message += text + ' ';
		}
		return message;
	};
}

times = 3;
var threeTimes = makeRepeater();
times = 4;
var fourTimes = makeRepeater();
alert( threeTimes('hi') );
// => 'hi hi hi hi '  ---> What?!?!
alert( fourTimes('hi') );
// => 'hi hi hi hi '

If it's not clear yet, let me write it down for you. The returned function really keeps a reference to any outside values it will need when invoked. In our original example, it kept a reference to the times local variable at the time it was produced. If we had created other local variables inside makeRepeater they would also become available inside the returned function. In other words, all the scope created during the call to makeRepeater will be preserved for the returned function. This happens when the returned (or inner) function has a reference to anything defined in the parent (or outer) function, i.e. the parent local scope. When this happens, we say that a closure has been created.

Closures can be tricky

It's important to understand the mechanics of closures to avoid subtle bugs in our code. Look at this piece of code, adapted from a real bug I had to fix.

<input type="button" value="Button 1" id="btn1">
<input type="button" value="Button 2" id="btn2">
<input type="button" value="Button 3" id="btn3">

<script type="text/javascript">
	function createEventHandlers(){
		var btn;
		for(var i=1; i <= 3; i++){
			btn = document.getElementById('btn' + i);
			btn.onclick = function(){
				alert('Clicked button #' + i);
			}
		}
	}
	createEventHandlers();
</script>

If you put this code in a page and click the three buttons you will see that all of them will show the message "Clicked button #4". Armed with our understanding of closures we can immediately understand that this bug is being caused by that reference to i used inside the event handler. We can fix that.

function createEventHandlers(){
	var btn;
	for(var i=1; i <= 3; i++){
		btn = document.getElementById('btn' + i);
		btn.onclick = createOneHandler(i);
	}
}

function createOneHandler(number){
	return function() {
		alert('Clicked button #' + number);
	}
}

The above code works because we are not creating functions inside the for loop, hence not producing closures on the same local scope. There is a different set of closures being produced by createOneHandler, but those are not pointing to the same parent scope. Each of these three new closures contain a different scope, created by each call to createOneHandler

Closing thoughts

Closures, of course, are not an exclusive feature of JavaScript. It's a very important trait of functional languages. Even in C#, when we use lambdas, closures are created — many times without us noticing.

The key to properly using closures in our code is to pay attention to locally scoped values from the outer function being used in the body of the inner function. Most of the times this will work as intended by the developer but, when it doesn't, stop and check if more than one closure is sharing the same local scope or if these local values are changing between the inner function creation and its invocation.