you - How do JavaScript closures work?




why we need closure in javascript (20)

How would you explain JavaScript closures to someone with a knowledge of the concepts they consist of (for example functions, variables and the like), but does not understand closures themselves?

I have seen the Scheme example given on Wikipedia, but unfortunately it did not help.


The children will always remember the secrets they have shared with their parents, even after their parents are gone. This is what closures are for functions.

The secrets for JavaScript functions are the private variables

var parent = function() {
 var name = "Mary"; // secret
}

Every time you call it, local variable "name" is created and given name "Mary". And every time the function exits the variable is lost and the name is forgotten.

As you may guess, because the variables are re-created every time the function is called, and nobody else will know them, there must be a secret place where they are stored. It could be called Chamber of Secrets or stack or local scope but it doesn't really matter. We know they are there, somewhere, hidden in the memory.

But, in JavaScript there is this very special thing that functions which are created inside other functions, can also know the local variables of their parents and keep them as long as they live.

var parent = function() {
  var name = "Mary";
  var child = function(childName) {
    // I can also see that "name" is "Mary"
  }
}

So, as long as we are in the parent -function, it can create one or more child functions which do share the secret variables from the secret place.

But the sad thing is, if the child is also a private variable of its parent function, it would also die when the parent ends, and the secrets would die with them.

So to live, the child has to leave before it's too late

var parent = function() {
  var name = "Mary";
  var child = function(childName) {
    return "My name is " + childName  +", child of " + name; 
  }
  return child; // child leaves the parent ->
}
var child = parent(); // < - and here it is outside 

And now, even though Mary is "no longer running", the memory of her is not lost and her child will always remember her name and other secrets they shared during their time together.

So, if you call the child "Alice", she will respond

child("Alice") => "My name is Alice, child of Mary"

That's all there is to tell.


JavaScript closures for beginners

Submitted by Morris on Tue, 2006-02-21 10:19. Community-edited since.

Closures are not magic

This page explains closures so that a programmer can understand them — using working JavaScript code. It is not for gurus or functional programmers.

Closures are not hard to understand once the core concept is grokked. However, they are impossible to understand by reading any theoretical or academically oriented explanations!

This article is intended for programmers with some programming experience in a mainstream language, and who can read the following JavaScript function:

function sayHello(name) {
  var text = 'Hello ' + name;
  var say = function() { console.log(text); }
  say();
}
sayHello('Joe');

Two brief summaries

  • When a function (foo) declares other functions (bar and baz), the family of local variables created in foo is not destroyed when the function exits. The variables merely become invisible to the outside world. Foo can therefore cunningly return the functions bar and baz, and they can continue to read, write and communicate with each other through this closed-off family of variables ("the closure") that nobody else can meddle with, not even someone who calls foo again in future.

  • A closure is one way of supporting first-class functions; it is an expression that can reference variables within its scope (when it was first declared), be assigned to a variable, be passed as an argument to a function, or be returned as a function result.

An example of a closure

The following code returns a reference to a function:

function sayHello2(name) {
  var text = 'Hello ' + name; // Local variable
  var say = function() { console.log(text); }
  return say;
}
var say2 = sayHello2('Bob');
say2(); // logs "Hello Bob"

Most JavaScript programmers will understand how a reference to a function is returned to a variable (say2) in the above code. If you don't, then you need to look at that before you can learn closures. A programmer using C would think of the function as returning a pointer to a function, and that the variables say and say2 were each a pointer to a function.

There is a critical difference between a C pointer to a function and a JavaScript reference to a function. In JavaScript, you can think of a function reference variable as having both a pointer to a function as well as a hidden pointer to a closure.

The above code has a closure because the anonymous function function() { console.log(text); } is declared inside another function, sayHello2() in this example. In JavaScript, if you use the function keyword inside another function, you are creating a closure.

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. This is demonstrated above, because we call the function say2() after we have returned from sayHello2(). Notice that the code that we call references the variable text, which was a local variable of the function sayHello2().

function() { console.log(text); } // Output of say2.toString();

Looking at the output of say2.toString(), We can see that the code refers to the variable text. The anonymous function can reference text which holds the value 'Hello Bob' because the local variables of sayHello2() have been secretly kept alive in a closure.

The genius is that in JavaScript a function reference also has a secret reference to the closure it was created in — similar to how delegates are a method pointer plus a secret reference to an object.

More examples

For some reason, closures seem really hard to understand when you read about them, but when you see some examples, it becomes clear how they work (it took me a while). I recommend working through the examples carefully until you understand how they work. If you start using closures without fully understanding how they work, you would soon create some very weird bugs!

Example 3

This example shows that the local variables are not copied — they are kept by reference. It is as though the stack-frame stays alive in memory even after the outer function exists!

function say667() {
  // Local variable that ends up within closure
  var num = 42;
  var say = function() { console.log(num); }
  num++;
  return say;
}
var sayNumber = say667();
sayNumber(); // logs 43

Example 4

All three global functions have a common reference to the same closure because they are all declared within a single call to setupSomeGlobals().

var gLogNumber, gIncreaseNumber, gSetNumber;
function setupSomeGlobals() {
  // Local variable that ends up within closure
  var num = 42;
  // Store some references to functions as global variables
  gLogNumber = function() { console.log(num); }
  gIncreaseNumber = function() { num++; }
  gSetNumber = function(x) { num = x; }
}

setupSomeGlobals();
gIncreaseNumber();
gLogNumber(); // 43
gSetNumber(5);
gLogNumber(); // 5

var oldLog = gLogNumber;

setupSomeGlobals();
gLogNumber(); // 42

oldLog() // 5

The three functions have shared access to the same closure — the local variables of setupSomeGlobals() when the three functions were defined.

Note that in the above example, if you call setupSomeGlobals() again, then a new closure (stack-frame!) is created. The old gLogNumber, gIncreaseNumber, gSetNumber variables are overwritten with new functions that have the new closure. (In JavaScript, whenever you declare a function inside another function, the inside function(s) is/are recreated again each time the outside function is called.)

Example 5

This example shows that the closure contains any local variables that were declared inside the outer function before it exited. Note that the variable alice is actually declared after the anonymous function. The anonymous function is declared first, and when that function is called it can access the alice variable because alice is in the same scope (JavaScript does variable hoisting). Also sayAlice()() just directly calls the function reference returned from sayAlice() — it is exactly the same as what was done previously but without the temporary variable.

function sayAlice() {
    var say = function() { console.log(alice); }
    // Local variable that ends up within closure
    var alice = 'Hello Alice';
    return say;
}
sayAlice()();// logs "Hello Alice"

Tricky: also note that the say variable is also inside the closure, and could be accessed by any other function that might be declared within sayAlice(), or it could be accessed recursively within the inside function.

Example 6

This one is a real gotcha for many people, so you need to understand it. Be very careful if you are defining a function within a loop: the local variables from the closure may not act as you might first think.

You need to understand the "variable hoisting" feature in Javascript in order to understand this example.

function buildList(list) {
    var result = [];
    for (var i = 0; i < list.length; i++) {
        var item = 'item' + i;
        result.push( function() {console.log(item + ' ' + list[i])} );
    }
    return result;
}

function testList() {
    var fnlist = buildList([1,2,3]);
    // Using j only to help prevent confusion -- could use i.
    for (var j = 0; j < fnlist.length; j++) {
        fnlist[j]();
    }
}

 testList() //logs "item2 undefined" 3 times

The line result.push( function() {console.log(item + ' ' + list[i])} adds a reference to an anonymous function three times to the result array. If you are not so familiar with anonymous functions think of it like:

pointer = function() {console.log(item + ' ' + list[i])};
result.push(pointer);

Note that when you run the example, "item2 undefined" is logged three times! This is because just like previous examples, there is only one closure for the local variables for buildList (which are result, i and item). When the anonymous functions are called on the line fnlist[j](); they all use the same single closure, and they use the current value for i and item within that one closure (where i has a value of 3 because the loop had completed, and item has a value of 'item2'). Note we are indexing from 0 hence item has a value of item2. And the i++ will increment i to the value 3.

It may be helpful to see what happens when a block-level declaration of the variable item is used (via the let keyword) instead of a function-scoped variable declaration via the var keyword. If that change is made, then each anonymous function in the array result has its own closure; when the example is run the output is as follows:

item0 undefined
item1 undefined
item2 undefined

If the variable i is also defined using let instead of var, then the output is:

item0 1
item1 2
item2 3

Example 7

In this final example, each call to the main function creates a separate closure.

function newClosure(someNum, someRef) {
    // Local variables that end up within closure
    var num = someNum;
    var anArray = [1,2,3];
    var ref = someRef;
    return function(x) {
        num += x;
        anArray.push(num);
        console.log('num: ' + num +
            '; anArray: ' + anArray.toString() +
            '; ref.someVar: ' + ref.someVar + ';');
      }
}
obj = {someVar: 4};
fn1 = newClosure(4, obj);
fn2 = newClosure(5, obj);
fn1(1); // num: 5; anArray: 1,2,3,5; ref.someVar: 4;
fn2(1); // num: 6; anArray: 1,2,3,6; ref.someVar: 4;
obj.someVar++;
fn1(2); // num: 7; anArray: 1,2,3,5,7; ref.someVar: 5;
fn2(2); // num: 8; anArray: 1,2,3,6,8; ref.someVar: 5;

Summary

If everything seems completely unclear, then the best thing to do is to play with the examples. Reading an explanation is much harder than understanding examples. My explanations of closures and stack-frames, etc. are not technically correct — they are gross simplifications intended to help to understand. Once the basic idea is grokked, you can pick up the details later.

Final points:

  • Whenever you use function inside another function, a closure is used.
  • Whenever you use eval() inside a function, a closure is used. The text you eval can reference local variables of the function, and within eval 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 create a closure. (The new function cannot reference the local variables of the outer function.)
  • A closure in JavaScript is like keeping a copy of all the local variables, just as they were when a function exited.
  • It is probably best to think that a closure is always created just an entry to a function, and the local variables are added to that closure.
  • A new set of local variables is kept every time a function with a closure is called (given that the function contains a function declaration inside it, and a reference to that inside function is either returned or an external reference is kept for it in some way).
  • Two functions might look like they have the same source text, but have completely different behavior because of their 'hidden' closure. I don't think JavaScript code can actually find out if a function reference has a closure or not.
  • If you are trying to do any dynamic source code modifications (for example: myFunction = Function(myFunction.toString().replace(/Hello/,'Hola'));), it won't work if myFunction is a closure (of course, you would never even think of doing source code string substitution at runtime, but...).
  • It is possible to get function declarations within function declarations within functions &mdash, and you can get closures at more than one level.
  • I think normally a closure is a term for both the function along with the variables that are captured. Note that I do not use that definition in this article!
  • I suspect that closures in JavaScript differ from those normally found in functional languages.

Links

Thanks

If you have just learned closures (here or elsewhere!), then I am interested in any feedback from you about any changes you might suggest that could make this article clearer. Send an email to morrisjohns.com (morris_closure @). Please note that I am not a guru on JavaScript — nor on closures.


Original post by Morris can be found in the Internet Archive.


Can you explain closures to a 5-year-old?*

I still think Google's explanation works very well and is concise:

/*
*    When a function is defined in another function and it
*    has access to the outer function's context even after
*    the outer function returns.
*
* An important concept to learn in JavaScript.
*/

function outerFunction(someNum) {
    var someString = 'Hey!';
    var content = document.getElementById('content');
    function innerFunction() {
        content.innerHTML = someNum + ': ' + someString;
        content = null; // Internet Explorer memory leak for DOM reference
    }
    innerFunction();
}

outerFunction(1);​

*A C# question


Closures are simple:

The following simple example covers all the main points of JavaScript closures.*  

Here is a factory that produces calculators that can add and multiply:

function make_calculator() {
  var n = 0; // this calculator stores a single number n
  return {
    add: function(a) {
      n += a;
      return n;
    },
    multiply: function(a) {
      n *= a;
      return n;
    }
  };
}

first_calculator = make_calculator();
second_calculator = make_calculator();

first_calculator.add(3); // returns 3
second_calculator.add(400); // returns 400

first_calculator.multiply(11); // returns 33
second_calculator.multiply(10); // returns 4000

The key point: Each call to make_calculator creates a new local variable n, which continues to be usable by that calculator's add and multiply functions long after make_calculator returns.

If you are familiar with stack frames, these calculators seem strange: How can they keep accessing n after make_calculator returns? The answer is to imagine that JavaScript doesn't use "stack frames", but instead uses "heap frames", which can persist after the function call that made them returns.

Inner functions like add and multiply, which access variables declared in an outer function**, are called closures.

That is pretty much all there is to closures.



* For example, it covers all the points in the "Closures for Dummies" article given in another answer, except example 6, which simply shows that variables can be used before they are declared, a nice fact to know but completely unrelated to closures. It also covers all the points in the accepted answer, except for the points (1) that functions copy their arguments into local variables (the named function arguments), and (2) that copying numbers creates a new number, but copying an object reference gives you another reference to the same object. These are also good to know but again completely unrelated to closures. It is also very similar to the example in this answer but a bit shorter and less abstract. It does not cover the point of this answer or this comment, which is that JavaScript makes it difficult to plug the current value of a loop variable into your inner function: The "plugging in" step can only be done with a helper function that encloses your inner function and is invoked on each loop iteration. (Strictly speaking, the inner function accesses the helper function's copy of the variable, rather than having anything plugged in.) Again, very useful when creating closures, but not part of what a closure is or how it works. There is additional confusion due to closures working differently in functional languages like ML, where variables are bound to values rather than to storage space, providing a constant stream of people who understand closures in a way (namely the "plugging in" way) that is simply incorrect for JavaScript, where variables are always bound to storage space, and never to values.

** Any outer function, if several are nested, or even in the global context, as this answer points out clearly.


I do not understand why the answers are so complex here.

Here is a closure:

var a = 42;

function b() { return a; }

Yes. You probably use that many times a day.


There is no reason to believe closures are a complex design hack to address specific problems. No, closures are just about using a variable that comes from a higher scope from the perspective of where the function was declared (not run).

Now what it allows you to do can be more spectacular, see other answers.


A closure is much like an object. It gets instantiated whenever you call a function.

The scope of a closure in JavaScript is lexical, which means that everything that is contained within the function the closure belongs to, has access to any variable that is in it.

A variable is contained in the closure if you

  1. assign it with var foo=1; or
  2. just write var foo;

If an inner function (a function contained inside another function) accesses such a variable without defining it in its own scope with var, it modifies the content of the variable in the outer closure.

A closure outlives the runtime of the function that spawned it. If other functions make it out of the closure/scope in which they are defined (for instance as return values), those will continue to reference that closure.

Example

function example(closure) {
  // define somevariable to live in the closure of example
  var somevariable = 'unchanged';

  return {
    change_to: function(value) {
      somevariable = value;
    },
    log: function(value) {
      console.log('somevariable of closure %s is: %s',
        closure, somevariable);
    }
  }
}

closure_one = example('one');
closure_two = example('two');

closure_one.log();
closure_two.log();
closure_one.change_to('some new value');
closure_one.log();
closure_two.log();

Output

somevariable of closure one is: unchanged
somevariable of closure two is: unchanged
somevariable of closure one is: some new value
somevariable of closure two is: unchanged

A function in JavaScript is not just a reference to a set of instructions (as in C language), but it also includes a hidden data structure which is composed of references to all nonlocal variables it uses (captured variables). Such two-piece functions are called closures. Every function in JavaScript can be considered a closure.

Closures are functions with a state. It is somewhat similar to "this" in the sense that "this" also provides state for a function but function and "this" are separate objects ("this" is just a fancy parameter, and the only way to bind it permanently to a function is to create a closure). While "this" and function always live separately, a function cannot be separated from its closure and the language provides no means to access captured variables.

Because all these external variables referenced by a lexically nested function are actually local variables in the chain of its lexically enclosing functions (global variables can be assumed to be local variables of some root function), and every single execution of a function creates new instances of its local variables, it follows that every execution of a function returning (or otherwise transferring it out, such as registering it as a callback) a nested function creates a new closure (with its own potentially unique set of referenced nonlocal variables which represent its execution context).

Also, it must be understood that local variables in JavaScript are created not on the stack frame, but on the heap and destroyed only when no one is referencing them. When a function returns, references to its local variables are decremented, but they can still be non-null if during the current execution they became part of a closure and are still referenced by its lexically nested functions (which can happen only if the references to these nested functions were returned or otherwise transferred to some external code).

An example:

function foo (initValue) {
   //This variable is not destroyed when the foo function exits.
   //It is 'captured' by the two nested functions returned below.
   var value = initValue;

   //Note that the two returned functions are created right now.
   //If the foo function is called again, it will return
   //new functions referencing a different 'value' variable.
   return {
       getValue: function () { return value; },
       setValue: function (newValue) { value = newValue; }
   }
}

function bar () {
    //foo sets its local variable 'value' to 5 and returns an object with
    //two functions still referencing that local variable
    var obj = foo(5);

    //Extracting functions just to show that no 'this' is involved here
    var getValue = obj.getValue;
    var setValue = obj.setValue;

    alert(getValue()); //Displays 5
    setValue(10);
    alert(getValue()); //Displays 10

    //At this point getValue and setValue functions are destroyed
    //(in reality they are destroyed at the next iteration of the garbage collector).
    //The local variable 'value' in the foo is no longer referenced by
    //anything and is destroyed too.
}

bar();

An answer for a six-year-old (assuming he knows what a function is and what a variable is, and what data is):

Functions can return data. One kind of data you can return from a function is another function. When that new function gets returned, all the variables and arguments used in the function that created it don't go away. Instead, that parent function "closes." In other words, nothing can look inside of it and see the variables it used except for the function it returned. That new function has a special ability to look back inside the function that created it and see the data inside of it.

function the_closure() {
  var x = 4;
  return function () {
    return x; // Here, we look back inside the_closure for the value of x
  }
}

var myFn = the_closure();
myFn(); //=> 4

Another really simple way to explain it is in terms of scope:

Any time you create a smaller scope inside of a larger scope, the smaller scope will always be able to see what is in the larger scope.


Closures are hard to explain because they are used to make some behaviour work that everybody intuitively expects to work anyway. I find the best way to explain them (and the way that I learned what they do) is to imagine the situation without them:

    var bind = function(x) {
        return function(y) { return x + y; };
    }
    
    var plus5 = bind(5);
    console.log(plus5(3));

What would happen here if JavaScript didn't know closures? Just replace the call in the last line by its method body (which is basically what function calls do) and you get:

console.log(x + 3);

Now, where's the definition of x? We didn't define it in the current scope. The only solution is to let plus5 carry its scope (or rather, its parent's scope) around. This way, x is well-defined and it is bound to the value 5.


Example for the first point by dlaliberte:

A closure is not only created when you return an inner function. In fact, the enclosing function does not need to return at all. You might instead assign your inner function to a variable in an outer scope, or pass it as an argument to another function where it could be used immediately. Therefore, the closure of the enclosing function probably already exists at the time that enclosing function was called since any inner function has access to it as soon as it is called.

var i;
function foo(x) {
    var tmp = 3;
    i = function (y) {
        console.log(x + y + (++tmp));
    }
}
foo(2);
i(3);

How I'd explain it to a six-year-old:

You know how grown-ups can own a house, and they call it home? When a mom has a child, the child doesn't really own anything, right? But its parents own a house, so whenever someone asks the child "Where's your home?", he/she can answer "that house!", and point to the house of its parents. A "Closure" is the ability of the child to always (even if abroad) be able to say it has a home, even though it's really the parent's who own the house.


I know there are plenty of solutions already, but I guess that this small and simple script can be useful to demonstrate the concept:

// makeSequencer will return a "sequencer" function
var makeSequencer = function() {
    var _count = 0; // not accessible outside this function
    var sequencer = function () {
        return _count++;
    }
    return sequencer;
}

var fnext = makeSequencer();
var v0 = fnext();     // v0 = 0;
var v1 = fnext();     // v1 = 1;
var vz = fnext._count // vz = undefined

I tend to learn better by GOOD/BAD comparisons. I like to see working code followed by non-working code that someone is likely to encounter. I put together a jsFiddle that does a comparison and tries to boil down the differences to the simplest explanations I could come up with.

Closures done right:

console.log('CLOSURES DONE RIGHT');

var arr = [];

function createClosure(n) {
    return function () {
        return 'n = ' + n;
    }
}

for (var index = 0; index < 10; index++) {
    arr[index] = createClosure(index);
}

for (var index in arr) {
    console.log(arr[index]());
}
  • In the above code createClosure(n) is invoked in every iteration of the loop. Note that I named the variable n to highlight that it is a new variable created in a new function scope and is not the same variable as index which is bound to the outer scope.

  • This creates a new scope and n is bound to that scope; this means we have 10 separate scopes, one for each iteration.

  • createClosure(n) returns a function that returns the n within that scope.

  • Within each scope n is bound to whatever value it had when createClosure(n) was invoked so the nested function that gets returned will always return the value of n that it had when createClosure(n) was invoked.

Closures done wrong:

console.log('CLOSURES DONE WRONG');

function createClosureArray() {
    var badArr = [];

    for (var index = 0; index < 10; index++) {
        badArr[index] = function () {
            return 'n = ' + index;
        };
    }
    return badArr;
}

var badArr = createClosureArray();

for (var index in badArr) {
    console.log(badArr[index]());
}
  • In the above code the loop was moved within the createClosureArray() function and the function now just returns the completed array, which at first glance seems more intuitive.

  • What might not be obvious is that since createClosureArray() is only invoked once only one scope is created for this function instead of one for every iteration of the loop.

  • Within this function a variable named index is defined. The loop runs and adds functions to the array that return index. Note that index is defined within the createClosureArray function which only ever gets invoked one time.

  • Because there was only one scope within the createClosureArray() function, index is only bound to a value within that scope. In other words, each time the loop changes the value of index, it changes it for everything that references it within that scope.

  • All of the functions added to the array return the SAME index variable from the parent scope where it was defined instead of 10 different ones from 10 different scopes like the first example. The end result is that all 10 functions return the same variable from the same scope.

  • After the loop finished and index was done being modified the end value was 10, therefore every function added to the array returns the value of the single index variable which is now set to 10.

Result

CLOSURES DONE RIGHT
n = 0
n = 1
n = 2
n = 3
n = 4
n = 5
n = 6
n = 7
n = 8
n = 9

CLOSURES DONE WRONG
n = 10
n = 10
n = 10
n = 10
n = 10
n = 10
n = 10
n = 10
n = 10
n = 10


I wrote a blog post a while back explaining closures. Here's what I said about closures in terms of why you'd want one.

Closures are a way to let a function have persistent, private variables - that is, variables that only one function knows about, where it can keep track of info from previous times that it was run.

In that sense, they let a function act a bit like an object with private attributes.

Full post:

So what are these closure thingys?


JavaScript functions can access their:

  1. Arguments
  2. Locals (that is, their local variables and local functions)
  3. Environment, which includes:
    • globals, including the DOM
    • anything in outer functions

If a function accesses its environment, then the function is a closure.

Note that outer functions are not required, though they do offer benefits I don't discuss here. By accessing data in its environment, a closure keeps that data alive. In the subcase of outer/inner functions, an outer function can create local data and eventually exit, and yet, if any inner function(s) survive after the outer function exits, then the inner function(s) keep the outer function's local data alive.

Example of a closure that uses the global environment:

Imagine that the Vote-Up and Vote-Down button events are implemented as closures, voteUp_click and voteDown_click, that have access to external variables isVotedUp and isVotedDown, which are defined globally. (For simplicity's sake, I am referring to 's Question Vote buttons, not the array of Answer Vote buttons.)

When the user clicks the VoteUp button, the voteUp_click function checks whether isVotedDown == true to determine whether to vote up or merely cancel a down vote. Function voteUp_click is a closure because it is accessing its environment.

var isVotedUp = false;
var isVotedDown = false;

function voteUp_click() {
  if (isVotedUp)
    return;
  else if (isVotedDown)
    SetDownVote(false);
  else
    SetUpVote(true);
}

function voteDown_click() {
  if (isVotedDown)
    return;
  else if (isVotedUp)
    SetUpVote(false);
  else
    SetDownVote(true);
}

function SetUpVote(status) {
  isVotedUp = status;
  // Do some CSS stuff to Vote-Up button
}

function SetDownVote(status) {
  isVotedDown = status;
  // Do some CSS stuff to Vote-Down button
}

All four of these functions are closures as they all access their environment.


OK, 6-year-old closures fan. Do you want to hear the simplest example of closure?

Let's imagine the next situation: a driver is sitting in a car. That car is inside a plane. Plane is in the airport. The ability of driver to access things outside his car, but inside the plane, even if that plane leaves an airport, is a closure. That's it. When you turn 27, look at the more detailed explanation or at the example below.

Here is how I can convert my plane story into the code.

var plane = function(defaultAirport) {

  var lastAirportLeft = defaultAirport;

  var car = {
    driver: {
      startAccessPlaneInfo: function() {
        setInterval(function() {
          console.log("Last airport was " + lastAirportLeft);
        }, 2000);
      }
    }
  };
  car.driver.startAccessPlaneInfo();

  return {
    leaveTheAirport: function(airPortName) {
      lastAirportLeft = airPortName;
    }
  }
}("Boryspil International Airport");

plane.leaveTheAirport("John F. Kennedy");

Perhaps a little beyond all but the most precocious of six-year-olds, but a few examples that helped make the concept of closure in JavaScript click for me.

A closure is a function that has access to another function's scope (its variables and functions). The easiest way to create a closure is with a function within a function; the reason being that in JavaScript a function always has access to its containing function’s scope.

function outerFunction() {
    var outerVar = "monkey";
    
    function innerFunction() {
        alert(outerVar);
    }
    
    innerFunction();
}

outerFunction();

ALERT: monkey

In the above example, outerFunction is called which in turn calls innerFunction. Note how outerVar is available to innerFunction, evidenced by its correctly alerting the value of outerVar.

Now consider the following:

function outerFunction() {
    var outerVar = "monkey";
    
    function innerFunction() {
        return outerVar;
    }
    
    return innerFunction;
}

var referenceToInnerFunction = outerFunction();
alert(referenceToInnerFunction());

ALERT: monkey

referenceToInnerFunction is set to outerFunction(), which simply returns a reference to innerFunction. When referenceToInnerFunction is called, it returns outerVar. Again, as above, this demonstrates that innerFunction has access to outerVar, a variable of outerFunction. Furthermore, it is interesting to note that it retains this access even after outerFunction has finished executing.

And here is where things get really interesting. If we were to get rid of outerFunction, say set it to null, you might think that referenceToInnerFunction would loose its access to the value of outerVar. But this is not the case.

function outerFunction() {
    var outerVar = "monkey";
    
    function innerFunction() {
        return outerVar;
    }
    
    return innerFunction;
}

var referenceToInnerFunction = outerFunction();
alert(referenceToInnerFunction());

outerFunction = null;
alert(referenceToInnerFunction());

ALERT: monkey ALERT: monkey

But how is this so? How can referenceToInnerFunction still know the value of outerVar now that outerFunction has been set to null?

The reason that referenceToInnerFunction can still access the value of outerVar is because when the closure was first created by placing innerFunction inside of outerFunction, innerFunction added a reference to outerFunction’s scope (its variables and functions) to its scope chain. What this means is that innerFunction has a pointer or reference to all of outerFunction’s variables, including outerVar. So even when outerFunction has finished executing, or even if it is deleted or set to null, the variables in its scope, like outerVar, stick around in memory because of the outstanding reference to them on the part of the innerFunction that has been returned to referenceToInnerFunction. To truly release outerVar and the rest of outerFunction’s variables from memory you would have to get rid of this outstanding reference to them, say by setting referenceToInnerFunction to null as well.

//////////

Two other things about closures to note. First, the closure will always have access to the last values of its containing function.

function outerFunction() {
    var outerVar = "monkey";
    
    function innerFunction() {
        alert(outerVar);
    }
    
    outerVar = "gorilla";

    innerFunction();
}

outerFunction();

ALERT: gorilla

Second, when a closure is created, it retains a reference to all of its enclosing function’s variables and functions; it doesn’t get to pick and choose. And but so, closures should be used sparingly, or at least carefully, as they can be memory intensive; a lot of variables can be kept in memory long after a containing function has finished executing.


Taking the question seriously, we should find out what a typical 6-year-old is capable of cognitively, though admittedly, one who is interested in JavaScript is not so typical.

On Childhood Development: 5 to 7 Years it says:

Your child will be able to follow two-step directions. For example, if you say to your child, "Go to the kitchen and get me a trash bag" they will be able to remember that direction.

We can use this example to explain closures, as follows:

The kitchen is a closure that has a local variable, called trashBags. There is a function inside the kitchen called getTrashBag that gets one trash bag and returns it.

We can code this in JavaScript like this:

function makeKitchen() {
  var trashBags = ['A', 'B', 'C']; // only 3 at first

  return {
    getTrashBag: function() {
      return trashBags.pop();
    }
  };
}

var kitchen = makeKitchen();

console.log(kitchen.getTrashBag()); // returns trash bag C
console.log(kitchen.getTrashBag()); // returns trash bag B
console.log(kitchen.getTrashBag()); // returns trash bag A

Further points that explain why closures are interesting:

  • Each time makeKitchen() is called, a new closure is created with its own separate trashBags.
  • The trashBags variable is local to the inside of each kitchen and is not accessible outside, but the inner function on the getTrashBag property does have access to it.
  • Every function call creates a closure, but there would be no need to keep the closure around unless an inner function, which has access to the inside of the closure, can be called from outside the closure. Returning the object with the getTrashBag function does that here.

This is an attempt to clear up several (possible) misunderstandings about closures that appear in some of the other answers.

  • A closure is not only created when you return an inner function. In fact, the enclosing function does not need to return at all in order for its closure to be created. You might instead assign your inner function to a variable in an outer scope, or pass it as an argument to another function where it could be called immediately or any time later. Therefore, the closure of the enclosing function is probably created as soon as the enclosing function is called since any inner function has access to that closure whenever the inner function is called, before or after the enclosing function returns.
  • A closure does not reference a copy of the old values of variables in its scope. The variables themselves are part of the closure, and so the value seen when accessing one of those variables is the latest value at the time it is accessed. This is why inner functions created inside of loops can be tricky, since each one has access to the same outer variables rather than grabbing a copy of the variables at the time the function is created or called.
  • The "variables" in a closure include any named functions declared within the function. They also include arguments of the function. A closure also has access to its containing closure's variables, all the way up to the global scope.
  • Closures use memory, but they don't cause memory leaks since JavaScript by itself cleans up its own circular structures that are not referenced. Internet Explorer memory leaks involving closures are created when it fails to disconnect DOM attribute values that reference closures, thus maintaining references to possibly circular structures.

Whenever you see the function keyword within another function, the inner function has access to variables in the outer function.

function foo(x) {
  var tmp = 3;

  function bar(y) {
    console.log(x + y + (++tmp)); // will log 16
  }

  bar(10);
}

foo(2);

This will always log 16, because bar can access the x which was defined as an argument to foo, and it can also access tmp from foo.

That is a closure. A function doesn't have to return in order to be called a closure. Simply accessing variables outside of your immediate lexical scope creates 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 is now a closure.
bar(10);

The above function will also log 16, because bar can still refer to x and tmp, even though it is no longer directly inside the scope.

However, since tmp is still hanging around inside bar's closure, it is also being 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 is created. Together with the function arguments and the parent object, this execution context also receives all the variables declared outside of it (in the above example, both 'a' and 'b').

It is possible to create more than one closure function, either by returning a list of them or by setting them to global variables. All of these will refer to the same x and the same tmp, they don't make their own copies.

Here the number x is a literal number. As with other literals in JavaScript, when foo is called, the number x is copied into foo as its argument x.

On the other hand, JavaScript always uses references when dealing with objects. If say, you called foo with an object, the closure it returns will reference that original object!

function foo(x) {
  var tmp = 3;

  return function (y) {
    console.log(x + y + tmp);
    x.memb = x.memb ? x.memb + 1 : 1;
    console.log(x.memb);
  }
}

var age = new Number(2);
var bar = foo(age); // bar is now a closure referencing age.
bar(10);

As expected, each call to bar(10) will increment x.memb. What might not be expected, is that x is simply referring to the same object as the age variable! After a couple of calls to bar, age.memb will be 2! This referencing is the basis for memory leaks with HTML objects.





closures