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)()) // 5
Event-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.counter
counter.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 43
Example 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() // 43
o.update(5)
o.log() // 5
const p = createObject()
p.log() // 42
Example 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
function
from 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 you eval
can reference local variables of the function, and in the non-strict mode, you can even create new local variables by using eval('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
Best Answer
EcmaScript 6 and up
If you're using ES6 or higher, the cleanest way is to construct an array of the items and use
Array.includes
:This has some inherent benefits over
indexOf
because it can properly test for the presence ofNaN
in the list, and can match missing array elements such as the middle one in[1, , 2]
toundefined
.includes
also works on JavaScript typed arrays such asUint8Array
.If you're concerned about browser support (such as for IE or Edge), you can check
Array.includes
at CanIUse.Com, and if you want to target a browser or browser version that's missingincludes
, I recommend polyfill.io for polyfilling.Higher Performance
Note that
Array.includes()
scales in its time with the number of elements in the array: it has performance O(n). If you need higher performance, and won't be constructing the set of items repeatedly (but will be repeatedly checking if the items contain some element), you should use aSet
.Note that you can pass in any iterable item to the
Set
constructor (anything that supports for...of). You can also convert aSet
to an array usingArray.from(set)
.Without An Array
You could add a new
isInList
property to strings as follows:Then use it like so:
You can do the same thing for
Number.prototype
.Note that
Object.defineProperty
cannot be used in IE8 and earlier, or very old versions of other browsers. However, it is a far superior solution toString.prototype.isInList = function() { ... }
because using simple assignment like that will create an enumerable property onString.prototype
, which is more likely to break code.Array.indexOf
If you are using a modern browser,
indexOf
always works. However, for IE8 and earlier you'll need a polyfill.If
indexOf
returns -1, the item is not in the list. Be mindful though, that this method will not properly check forNaN
, and while it can match an explicitundefined
, it can’t match a missing element toundefined
as in the array[1, , 2]
.Polyfill for
indexOf
orincludes
in IE, or any other browser/version lacking supportIf you don't want to use a service like polyfill.io as mentioned above, you can always include in your own source code standards-compliant custom polyfills. For example, Mozilla Developer Network has one for
indexOf
.In this situation where I had to make a solution for Internet Explorer 7, I "rolled my own" simpler version of the
indexOf()
function that is not standards-compliant:However, I don't think modifying
Array.prototype
is the best answer in the long term. ModifyingObject
andArray
prototypes in JavaScript can lead to serious bugs. You need to decide whether doing so is safe in your own environment. Of primary note is that iterating an array (when Array.prototype has added properties) withfor ... in
will return the new function name as one of the keys:Your code may work now, but the moment someone in the future adds a third-party JavaScript library or plugin that isn't zealously guarding against inherited keys, everything can break.
The old way to avoid that breakage is, during enumeration, to check each value to see if the object actually has it as a non-inherited property with
if (arr.hasOwnProperty(x))
and only then work with thatx
.The new ES6 ways to avoid this extra-key problem are:
Use
of
instead ofin
,for (let x of arr)
. However, unless you can guarantee that all of your code and third-party libraries strictly stick to this method, then for the purposes of this question you'll probably just want to useincludes
as stated above.Define your new properties on the prototype using
Object.defineProperty()
, as this will make the property (by default) non-enumerable. This only truly solves the problem if all the JavaScript libraries or modules you use also do this.