How do JavaScript closures work?
A closure is a pairing of:
- A function, and
- A reference to that function's outer scope (lexical environment)
A lexical environment is part of every execution context (stack frame) and is a map between identifiers (ie. local variable names) and values.
Every function in JavaScript maintains a reference to its outer lexical environment. This reference is used to configure the execution context created when a function is invoked. This reference enables code inside the function to "see" variables declared outside the function, regardless of when and where the function is called.
If a function was called by a function, which in turn was called by another function, then a chain of references to outer lexical environments is created. This chain is called the scope chain.
In the following code, inner forms a closure with the lexical environment of the execution context created when foo is invoked, closing over variable secret:
function foo() { const secret = Math.trunc(Math.random()*100) return function inner() { console.log(`The secret number is ${secret}.`) }}const f = foo() // `secret` is not directly accessible from outside `foo`f() // The only way to retrieve `secret`, is to invoke `f`In other words: in JavaScript, functions carry a reference to a private "box of state", to which only they (and any other functions declared within the same lexical environment) have access. This box of the state is invisible to the caller of the function, delivering an excellent mechanism for data-hiding and encapsulation.
And remember: functions in JavaScript can be passed around like variables (first-class functions), meaning these pairings of functionality and state can be passed around your program: similar to how you might pass an instance of a class around in C++.
If JavaScript did not have closures, then more states would have to be passed between functions explicitly, making parameter lists longer and code noisier.
So, if you want a function to always have access to a private piece of state, you can use a closure.
...and frequently we do want to associate the state with a function. For example, in Java or C++, when you add a private instance variable and a method to a class, you are associating state with functionality.
In C and most other common languages, after a function returns, all the local variables are no longer accessible because the stack-frame is destroyed. In JavaScript, if you declare a function within another function, then the local variables of the outer function can remain accessible after returning from it. In this way, in the code above, secret remains available to the function object inner, after it has been returned from foo.
Uses of Closures
Closures are useful whenever you need a private state associated with a function. This is a very common scenario - and remember: JavaScript did not have a class syntax until 2015, and it still does not have a private field syntax. Closures meet this need.
Private Instance Variables
In the following code, the function toString closes over the details of the car.
function Car(manufacturer, model, year, color) { return { toString() { return `${manufacturer} ${model} (${year}, ${color})` } }}const car = new Car('Aston Martin','V8 Vantage','2012','Quantum Silver')console.log(car.toString())Functional Programming
In the following code, the function inner closes over both fn and args.
function curry(fn) { const args = [] return function inner(arg) { if(args.length === fn.length) return fn(...args) args.push(arg) return inner }}function add(a, b) { return a + b}const curriedAdd = curry(add)console.log(curriedAdd(2)(3)()) // 5Event-Oriented Programming
In the following code, function onClick closes over variable BACKGROUND_COLOR.
const $ = document.querySelector.bind(document)const BACKGROUND_COLOR = 'rgba(200,200,242,1)'function onClick() { $('body').style.background = BACKGROUND_COLOR}$('button').addEventListener('click', onClick)<button>Set background color</button>Modularization
In the following example, all the implementation details are hidden inside an immediately executed function expression. The functions tick and toString close over the private state and functions they need to complete their work. Closures have enabled us to modularise and encapsulate our code.
let namespace = {};(function foo(n) { let numbers = [] function format(n) { return Math.trunc(n) } function tick() { numbers.push(Math.random() * 100) } function toString() { return numbers.map(format) } n.counter = { tick, toString }}(namespace))const counter = namespace.countercounter.tick()counter.tick()console.log(counter.toString())Examples
Example 1
This example shows that the local variables are not copied in the closure: the closure maintains a reference to the original variables themselves. It is as though the stack-frame stays alive in memory even after the outer function exits.
function foo() { let x = 42 let inner = function() { console.log(x) } x = x+1 return inner}var f = foo()f() // logs 43Example 2
In the following code, three methods log, increment, and update all close over the same lexical environment.
And every time createObject is called, a new execution context (stack frame) is created and a completely new variable x, and a new set of functions (log etc.) are created, that close over this new variable.
function createObject() { let x = 42; return { log() { console.log(x) }, increment() { x++ }, update(value) { x = value } }}const o = createObject()o.increment()o.log() // 43o.update(5)o.log() // 5const p = createObject()p.log() // 42Example 3
If you are using variables declared using var, be careful you understand which variable you are closing over. Variables declared using var are hoisted. This is much less of a problem in modern JavaScript due to the introduction of let and const.
In the following code, each time around the loop, a new function inner is created, which closes over i. But because var i is hoisted outside the loop, all of these inner functions close over the same variable, meaning that the final value of i (3) is printed, three times.
function foo() { var result = [] for (var i = 0; i < 3; i++) { result.push(function inner() { console.log(i) } ) } return result}const result = foo()// The following will print `3`, three times...for (var i = 0; i < 3; i++) { result[i]() }Final points:
- Whenever a function is declared in JavaScript closure is created.
- Returning a
functionfrom inside another function is the classic example of closure, because the state inside the outer function is implicitly available to the returned inner function, even after the outer function has completed execution. - Whenever you use
eval()inside a function, a closure is used. The text youevalcan reference local variables of the function, and in the non-strict mode, you can even create new local variables by usingeval('var foo = …'). - When you use
new Function(…)(the Function constructor) inside a function, it does not close over its lexical environment: it closes over the global context instead. The new function cannot reference the local variables of the outer function. - A closure in JavaScript is like keeping a reference (NOT a copy) to the scope at the point of function declaration, which in turn keeps a reference to its outer scope, and so on, all the way to the global object at the top of the scope chain.
- A closure is created when a function is declared; this closure is used to configure the execution context when the function is invoked.
- A new set of local variables is created every time a function is called.
Links
- Douglas Crockford's simulated private attributes and private methods for an object, using closures.
- A great explanation of how closures can cause memory leaks in IE if you are not careful.
- MDN documentation on JavaScript Closures.
Every function in JavaScript maintains a link to its outer lexical environment. A lexical environment is a map of all the names (eg. variables, parameters) within a scope, with their values.
So, whenever you see the function keyword, code inside that function has access to variables declared outside the function.
function foo(x) { var tmp = 3; function bar(y) { console.log(x + y + (++tmp)); // will log 16 } bar(10);}foo(2);This will log 16 because function bar closes over the parameter x and the variable tmp, both of which exist in the lexical environment of outer function foo.
Function bar, together with its link with the lexical environment of function foo is a closure.
A function doesn't have to return in order to create a closure. Simply by virtue of its declaration, every function closes over its enclosing lexical environment, forming a closure.
function foo(x) { var tmp = 3; return function (y) { console.log(x + y + (++tmp)); // will also log 16 }}var bar = foo(2);bar(10); // 16bar(10); // 17The above function will also log 16, because the code inside bar can still refer to argument x and variable tmp, even though they are no longer directly in scope.
However, since tmp is still hanging around inside bar's closure, it is available to be incremented. It will be incremented each time you call bar.
The simplest example of a closure is this:
var a = 10;function test() { console.log(a); // will output 10 console.log(b); // will output 6}var b = 6;test();When a JavaScript function is invoked, a new execution context ec is created. Together with the function arguments and the target object, this execution context also receives a link to the lexical environment of the calling execution context, meaning the variables declared in the outer lexical environment (in the above example, both a and b) are available from ec.
Every function creates a closure because every function has a link to its outer lexical environment.
Note that variables themselves are visible from within a closure, not copies.
FOREWORD: this answer was written when the question was:
Like the old Albert said : "If you can't explain it to a six-year old, you really don't understand it yourself.”. Well I tried to explain JS closures to a 27 years old friend and completely failed.
Can anybody consider that I am 6 and strangely interested in that subject ?
I'm pretty sure I was one of the only people that attempted to take the initial question literally. Since then, the question has mutated several times, so my answer may now seem incredibly silly & out of place. Hopefully the general idea of the story remains fun for some.
I'm a big fan of analogy and metaphor when explaining difficult concepts, so let me try my hand with a story.
Once upon a time:
There was a princess...
function princess() {She lived in a wonderful world full of adventures. She met her Prince Charming, rode around her world on a unicorn, battled dragons, encountered talking animals, and many other fantastical things.
var adventures = []; function princeCharming() { /* ... */ } var unicorn = { /* ... */ }, dragons = [ /* ... */ ], squirrel = "Hello!"; /* ... */But she would always have to return back to her dull world of chores and grown-ups.
return {And she would often tell them of her latest amazing adventure as a princess.
story: function() { return adventures[adventures.length - 1]; } };}But all they would see is a little girl...
var littleGirl = princess();...telling stories about magic and fantasy.
littleGirl.story();And even though the grown-ups knew of real princesses, they would never believe in the unicorns or dragons because they could never see them. The grown-ups said that they only existed inside the little girl's imagination.
But we know the real truth; that the little girl with the princess inside...
...is really a princess with a little girl inside.