A closure is a function bundled with its lexical environment
JavaScript is a lexically scoped language. This means that functions use the variable scope that was in effect when they were defined (not the variable scope in effect when they are invoked).
Technically, all JavaScript functions are closures, but because most functions are invoked from the same scope they were defined, it doesn't matter that there was a closure involved.
Closures are commonly used for encapsulation (the ability to have private properties for objects), functional programming (curried functions, partial applications) and to grant access to variables inside event listeners.
Let's take a look at each of these use cases to help us understand what closure is.
Encapsulation
Say we had a factory function that returned a counter object:
const counter = () => ({
n: 0,
count() { this.n++ },
reset() { this.n = 0 }
})
const counter1 = counter();
counter1.count();
counter1.count();
console.log(counter1.n) // 2
counter1.n = 0; // << We don't want this
console.log(counter1) // { n: 0, ... } uh oh!
Buggy or malicious code could reset the counter without calling the reset() method as shown above.
As mentioned in my post on encapsulation, this breaks a fundamental principle of good software design:
“Program to an interface, not an implementation.” — Gang of Four, “Design Patterns: Elements of Reusable Object-Oriented Software”
We only want to be able to communicate with counter by using its interface and by passing messages (methods) such as count() or reset(). We don't want to be able to reach in and manipulate properties such as n directly. Unfortunately, the property n forms part of the public interface for this object and so it is easily manipulated. Let's change that. Closure can help us out here. Take a look at this revised example:
const counter = () => {
let n = 0;
return {
count() { n++ },
reset() { n = 0 },
getCount() { console.log(n) }
}
}
const counter1 = counter();
counter1.count();
counter1.count();
counter1.getCount() // 2
console.log(counter1.n) // undefined
Before we dissect this. Reconsider our definition of closure - a function bundled with its lexical environment. The lexical environment being the variable scope that was in effect when the function was defined.
n is in scope when count, reset and getCount are defined and so, when counter returns and the object is created, the only code that will have direct access to n is this instance of the counter object and the methods on it.
Note that the reference to n is live and each invocation of counter creates a new scope independent of scopes created by previous invocations and a new private variable within that scope. So what is n for counter1 may not the what is n for counter2.
Partial application
A partial application is a function that has been applied some but not all of it's arguments. Let's look at an example:
const trace = label => value => {
console.log(`${ label }: ${ value }`);
};
trace is a function that takes a label and a value and logs it to the console.
Because this function is curried we can create specialist 'sub-functions' that are partial applications of the full trace function:
const traceLabelX = trace('Label X')
console.log(traceLabelX.toString()) // 'value => {console.log(`${label}: ${value}`);}'
traceLabelX(20) // 'Label X : 20'
If you log traceLabelX to the console you see it return a function that takes in a value and logs the label and the value. But where is label? This function's closure has access to the label it was returned with anywhere it is now used.
Event listeners
Open up VSCode and make this little .html page and open it up in a browser.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
</head>
<body>
Closures in event listeners
</body>
<script>
const body = document.body;
const initButtons = () => {
let button;
for (var i = 0; i < 5; i++) {
button = document.createElement("button");
button.innerHTML = "Button " + i;
button.addEventListener("click", (e) => {
alert(i);
});
body.appendChild(button);
}
};
initButtons();
</script>
</html>
What do you think happens when you click on the buttons? Each button click will return an alert with '5'. Why is this? The first thing to note here is that we are using var not let to declare i. As such this is a bit of a contrived example as you would very rarely use var for variable declaration these days but stick with me as it will help you understand closures. Remember - var is function scoped and let is block scoped.
The for loop is within the initButtons function and var is 'hoisted' to the top of the function.
Every time a loop completes a button is created with an attached event listener who's callback has reference to i. As subsequent loops complete, i updates, as too does each event-listeners reference to it. This is the problem, every closure has access to the same reference to i.
We could fix this in a couple of ways:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
</head>
<body>
Closures in event listeners
</body>
<script>
const body = document.body;
const initButton = (name, alertMessage) => {
button = document.createElement("button");
button.innerHTML = "Button " + name;
button.addEventListener("click", (e) => {
alert(alertMessage);
});
body.appendChild(button);
};
for (var i = 0; i < 5; i++) {
initButton(i, i);
}
</script>
</html>
Each event listener is now scoped to the alertMessage param which is defined on function invocation.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
</head>
<body>
Closures in event listeners
</body>
<script>
const body = document.body;
const initButtons = () => {
let button;
for (let i = 0; i < 5; i++) {
button = document.createElement("button");
button.innerHTML = "Button " + i;
button.addEventListener("click", (e) => {
alert(i);
});
body.appendChild(button);
}
};
initButtons();
</script>
</html>
Or just use let instead of var within the loop. Using let will ensure that each iteration of the scope has its own independent binding of i.
Has this helped you understand closure? Let me know in the comments!
References
- https://medium.com/javascript-scene/master-the-javascript-interview-what-is-a-closure-b2f0d2152b36#.11d4u33p7
- https://medium.com/javascript-scene/curry-and-function-composition-2c208d774983
- JavaScript: The Definitive Guide, 7th Edition by David Flanagan
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