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
Editor's Note: All functions in JavaScript are closures as explained in this post. However we are only interested in identifying a subset of these functions which are interesting from a theoretical point of view. Henceforth any reference to the word closure will refer to this subset of functions unless otherwise stated.
A simple explanation for closures:
- Take a function. Let's call it F.
- List all the variables of F.
- The variables may be of two types:
- Local variables (bound variables)
- Non-local variables (free variables)
- If F has no free variables then it cannot be a closure.
- If F has any free variables (which are defined in a parent scope of F) then:
- There must be only one parent scope of F to which a free variable is bound.
- If F is referenced from outside that parent scope, then it becomes a closure for that free variable.
- That free variable is called an upvalue of the closure F.
Now let's use this to figure out who uses closures and who doesn't (for the sake of explanation I have named the functions):
Case 1: Your Friend's Program
for (var i = 0; i < 10; i++) {
(function f() {
var i2 = i;
setTimeout(function g() {
console.log(i2);
}, 1000);
})();
}
In the above program there are two functions: f and g. Let's see if they are closures:
For f:
- List the variables:
i2 is a local variable.i is a free variable.setTimeout is a free variable.g is a local variable.console is a free variable.
- Find the parent scope to which each free variable is bound:
i is bound to the global scope.setTimeout is bound to the global scope.console is bound to the global scope.
- In which scope is the function referenced? The global scope.
- Hence
i is not closed over by f.
- Hence
setTimeout is not closed over by f.
- Hence
console is not closed over by f.
Thus the function f is not a closure.
For g:
- List the variables:
console is a free variable.i2 is a free variable.
- Find the parent scope to which each free variable is bound:
console is bound to the global scope.i2 is bound to the scope of f.
- In which scope is the function referenced? The scope of
setTimeout.
- Hence
console is not closed over by g.
- Hence
i2 is closed over by g.
Thus the function g is a closure for the free variable i2 (which is an upvalue for g) when it's referenced from within setTimeout.
Bad for you: Your friend is using a closure. The inner function is a closure.
Case 2: Your Program
for (var i = 0; i < 10; i++) {
setTimeout((function f(i2) {
return function g() {
console.log(i2);
};
})(i), 1000);
}
In the above program there are two functions: f and g. Let's see if they are closures:
For f:
- List the variables:
i2 is a local variable.g is a local variable.console is a free variable.
- Find the parent scope to which each free variable is bound:
console is bound to the global scope.
- In which scope is the function referenced? The global scope.
- Hence
console is not closed over by f.
Thus the function f is not a closure.
For g:
- List the variables:
console is a free variable.i2 is a free variable.
- Find the parent scope to which each free variable is bound:
console is bound to the global scope.i2 is bound to the scope of f.
- In which scope is the function referenced? The scope of
setTimeout.
- Hence
console is not closed over by g.
- Hence
i2 is closed over by g.
Thus the function g is a closure for the free variable i2 (which is an upvalue for g) when it's referenced from within setTimeout.
Good for you: You are using a closure. The inner function is a closure.
So both you and your friend are using closures. Stop arguing. I hope I cleared the concept of closures and how to identify them for the both of you.
Edit: A simple explanation as to why are all functions closures (credits @Peter):
First let's consider the following program (it's the control):
lexicalScope();
function lexicalScope() {
var message = "This is the control. You should be able to see this message being alerted.";
regularFunction();
function regularFunction() {
alert(eval("message"));
}
}
- We know that both
lexicalScope and regularFunction aren't closures from the above definition.
- When we execute the program we expect
message to be alerted because regularFunction is not a closure (i.e. it has access to all the variables in its parent scope - including message).
- When we execute the program we observe that
message is indeed alerted.
Next let's consider the following program (it's the alternative):
var closureFunction = lexicalScope();
closureFunction();
function lexicalScope() {
var message = "This is the alternative. If you see this message being alerted then in means that every function in JavaScript is a closure.";
return function closureFunction() {
alert(eval("message"));
};
}
- We know that only
closureFunction is a closure from the above definition.
- When we execute the program we expect
message not to be alerted because closureFunction is a closure (i.e. it only has access to all its non-local variables at the time the function is created (see this answer) - this does not include message).
- When we execute the program we observe that
message is actually being alerted.
What do we infer from this?
- JavaScript interpreters do not treat closures differently from the way they treat other functions.
- Every function carries its scope chain along with it. Closures don't have a separate referencing environment.
- A closure is just like every other function. We just call them closures when they are referenced in a scope outside the scope to which they belong because this is an interesting case.
Best Answer
If you assign a closure to a property of a class instance, and the closure captures that instance by referring to the instance or its members, you will create a strong reference cycle between the closure and the instance. Swift uses capture lists to break these strong reference cycles. source Apple
source sketchyTech First, it is important to make clear that this whole issue only concerns closures where we are assigning "a closure to a property of a class instance". Keep this in mind with each rule. The rules:
In answear to your question there should be no retain cycle.