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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.
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 (i.e. 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 the 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 modularize 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 = () => console.log(x)
x = x + 1
return inner
}
foo()() // 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
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); // 16
bar(10); // 17
The 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.
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.
The Straw Man
I need to know how many times a button has been clicked and do something on every third click...
Fairly Obvious Solution
// Declare counter outside event handler's scope
var counter = 0;
var element = document.getElementById('button');
element.addEventListener("click", function() {
// Increment outside counter
counter++;
if (counter === 3) {
// Do something every third time
console.log("Third time's the charm!");
// Reset counter
counter = 0;
}
});
<button id="button">Click Me!</button>
Now this will work, but it does encroach into the outer scope by adding a variable, whose sole purpose is to keep track of the count. In some situations, this would be preferable as your outer application might need access to this information. But in this case, we are only changing every third click's behavior, so it is preferable to enclose this functionality inside the event handler.
Consider this option
var element = document.getElementById('button');
element.addEventListener("click", (function() {
// init the count to 0
var count = 0;
return function(e) { // <- This function becomes the click handler
count++; // and will retain access to the above `count`
if (count === 3) {
// Do something every third time
console.log("Third time's the charm!");
//Reset counter
count = 0;
}
};
})());
<button id="button">Click Me!</button>
Notice a few things here.
In the above example, I am using the closure behavior of JavaScript. This behavior allows any function to have access to the scope in which it was created, indefinitely. To practically apply this, I immediately invoke a function that returns another function, and because the function I'm returning has access to the internal count variable (because of the closure behavior explained above) this results in a private scope for usage by the resulting function... Not so simple? Let's dilute it down...
A simple one-line closure
// _______________________Immediately invoked______________________
// | |
// | Scope retained for use ___Returned as the____ |
// | only by returned function | value of func | |
// | | | | | |
// v v v v v v
var func = (function() { var a = 'val'; return function() { alert(a); }; })();
All variables outside the returned function are available to the returned function, but they are not directly available to the returned function object...
func(); // Alerts "val"
func.a; // Undefined
Get it? So in our primary example, the count variable is contained within the closure and always available to the event handler, so it retains its state from click to click.
Also, this private variable state is fully accessible, for both readings and assigning to its private scoped variables.
There you go; you're now fully encapsulating this behavior.
Full Blog Post (including jQuery considerations)
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:
const makePlus = function(x) {
return function(y) { return x + y; };
}
const plus5 = makePlus(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.
TLDR
A closure is a link between a function and its outer lexical (ie. as-written) environment, such that the identifiers (variables, parameters, function declarations etc) defined within that environment are visible from within the function, regardless of when or from where the function is invoked.
Details
In the terminology of the ECMAScript specification, a closure can be said to be implemented by the [[Environment]] reference of every function-object, which points to the lexical environment within which the function is defined.
When a function is invoked via the internal [[Call]] method, the [[Environment]] reference on the function-object is copied into the outer environment reference of the environment record of the newly-created execution context (stack frame).
In the following example, function f closes over the lexical environment of the global execution context:
function f() {}
In the following example, function h closes over the lexical environment of function g, which, in turn, closes over the lexical environment of the global execution context.
function g() {
function h() {}
}
If an inner function is returned by an outer, then the outer lexical environment will persist after the outer function has returned. This is because the outer lexical environment needs to be available if the inner function is eventually invoked.
In the following example, function j closes over the lexical environment of function i, meaning that variable x is visible from inside function j, long after function i has completed execution:
function i() {
var x = 'mochacchino'
return function j() {
console.log('Printing the value of x, from within function j: ', x)
}
}
const k = i()
setTimeout(k, 500) // invoke k (which is j) after 500ms
In a closure, the variables in the outer lexical environment themselves are available, not copies.
function l() {
var y = 'vanilla';
return {
setY: function(value) {
y = value;
},
logY: function(value) {
console.log('The value of y is: ', y);
}
}
}
const o = l()
o.logY() // The value of y is: vanilla
o.setY('chocolate')
o.logY() // The value of y is: chocolate
The chain of lexical environments, linked between execution contexts via outer environment references, forms a scope chain and defines the identifiers visible from any given function.
Please note that in an attempt to improve clarity and accuracy, this answer has been substantially changed from the original.
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");
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.
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?
The original question had a quote:
If you can't explain it to a six-year old, you really don't understand it yourself.
This is how I'd try to explain it to an actual 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 "Where's your home?", the child 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 refer to its home, even though it's really the parent's who own the house.
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.
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
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 of 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 of 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
Wikipedia on closures:
In computer science, a closure is a function together with a referencing environment for the nonlocal names (free variables) of that function.
Technically, in JavaScript, every function is a closure. It always has an access to variables defined in the surrounding scope.
Since scope-defining construction in JavaScript is a function, not a code block like in many other languages, what we usually mean by closure in JavaScript is a function working with nonlocal variables defined in already executed surrounding function.
Closures are often used for creating functions with some hidden private data (but it's not always the case).
var db = (function() {
// Create a hidden object, which will hold the data
// it's inaccessible from the outside.
var data = {};
// Make a function, which will provide some access to the data.
return function(key, val) {
if (val === undefined) { return data[key] } // Get
else { return data[key] = val } // Set
}
// We are calling the anonymous surrounding function,
// returning the above inner function, which is a closure.
})();
db('x') // -> undefined
db('x', 1) // Set x to 1
db('x') // -> 1
// It's impossible to access the data object itself.
// We are able to get or set individual it.
ems
The example above is using an anonymous function, which was executed once. But it does not have to be. It can be named (e.g. mkdb) and executed later, generating a database function each time it is invoked. Every generated function will have its own hidden database object. Another usage example of closures is when we don't return a function, but an object containing multiple functions for different purposes, each of those function having access to the same data.
The children will never forget 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, the local variable "name" is created and given the 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 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.
I put together an interactive JavaScript tutorial to explain how closures work.
What's a Closure?
Here's one of the examples:
var create = function (x) {
var f = function () {
return x; // We can refer to x here!
};
return f;
};
// 'create' takes one argument, creates a function
var g = create(42);
// g is a function that takes no arguments now
var y = g();
// y is 42 here
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 where an inner function has access to variables in its outer function. That's probably the simplest one-line explanation you can get for closures.
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);
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
You're having a sleep over and you invite Dan.
You tell Dan to bring one XBox controller.
Dan invites Paul.
Dan asks Paul to bring one controller. How many controllers were brought to the party?
function sleepOver(howManyControllersToBring) {
var numberOfDansControllers = howManyControllersToBring;
return function danInvitedPaul(numberOfPaulsControllers) {
var totalControllers = numberOfDansControllers + numberOfPaulsControllers;
return totalControllers;
}
}
var howManyControllersToBring = 1;
var inviteDan = sleepOver(howManyControllersToBring);
// The only reason Paul was invited is because Dan was invited.
// So we set Paul's invitation = Dan's invitation.
var danInvitedPaul = inviteDan(howManyControllersToBring);
alert("There were " + danInvitedPaul + " controllers brought to the party.");
The author of Closures has explained closures pretty well, explaining the reason why we need them and also explaining LexicalEnvironment which is necessary to understanding closures.
Here is the summary:
What if a variable is accessed, but it isn’t local? Like here:
In this case, the interpreter finds the variable in the
outer LexicalEnvironment object.
The process consists of two steps:
First, when a function f is created, it is not created in an empty
space. There is a current LexicalEnvironment object. In the case
above, it’s window (a is undefined at the time of function
creation).
When a function is created, it gets a hidden property, named [[Scope]], which references the current LexicalEnvironment.
If a variable is read, but can not be found anywhere, an error is generated.
Nested functions
Functions can be nested one inside another, forming a chain of LexicalEnvironments which can also be called a scope chain.
So, function g has access to g, a and f.
Closures
A nested function may continue to live after the outer function has finished:
Marking up LexicalEnvironments:
As we see, this.say is a property in the user object, so it continues to live after User completed.
And if you remember, when this.say is created, it (as every function) gets an internal reference this.say.[[Scope]] to the current LexicalEnvironment. So, the LexicalEnvironment of the current User execution stays in memory. All variables of User also are its properties, so they are also carefully kept, not junked as usually.
The whole point is to ensure that if the inner function wants to access an outer variable in the future, it is able to do so.
To summarize:
The inner function keeps a reference to the outer
LexicalEnvironment.
The inner function may access variables from it
any time even if the outer function is finished.
The browser keeps the LexicalEnvironment and all its properties (variables) in memory until there is an inner function which references it.
This is called a closure.
JavaScript functions can access their:
Arguments
Locals (that is, their local variables and local functions)
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 Stack Overflow 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 StackOverflow'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.
As a father of a 6-year-old, currently teaching young children (and a relative novice to coding with no formal education so corrections will be required), I think the lesson would stick best through hands-on play. If the 6-year-old is ready to understand what a closure is, then they are old enough to have a go themselves. I'd suggest pasting the code into jsfiddle.net, explaining a bit, and leaving them alone to concoct a unique song. The explanatory text below is probably more appropriate for a 10 year old.
function sing(person) {
var firstPart = "There was " + person + " who swallowed ";
var fly = function() {
var creature = "a fly";
var result = "Perhaps she'll die";
alert(firstPart + creature + "\n" + result);
};
var spider = function() {
var creature = "a spider";
var result = "that wiggled and jiggled and tickled inside her";
alert(firstPart + creature + "\n" + result);
};
var bird = function() {
var creature = "a bird";
var result = "How absurd!";
alert(firstPart + creature + "\n" + result);
};
var cat = function() {
var creature = "a cat";
var result = "Imagine That!";
alert(firstPart + creature + "\n" + result);
};
fly();
spider();
bird();
cat();
}
var person="an old lady";
sing(person);
INSTRUCTIONS
DATA: Data is a collection of facts. It can be numbers, words, measurements, observations or even just descriptions of things. You can't touch it, smell it or taste it. You can write it down, speak it and hear it. You could use it to create touch smell and taste using a computer. It can be made useful by a computer using code.
CODE: All the writing above is called code. It is written in JavaScript.
JAVASCRIPT: JavaScript is a language. Like English or French or Chinese are languages. There are lots of languages that are understood by computers and other electronic processors. For JavaScript to be understood by a computer it needs an interpreter. Imagine if a teacher who only speaks Russian comes to teach your class at school. When the teacher says "все садятся", the class would not understand. But luckily you have a Russian pupil in your class who tells everyone this means "everybody sit down" - so you all do. The class is like a computer and the Russian pupil is the interpreter. For JavaScript the most common interpreter is called a browser.
BROWSER: When you connect to the Internet on a computer, tablet or phone to visit a website, you use a browser. Examples you may know are Internet Explorer, Chrome, Firefox and Safari. The browser can understand JavaScript and tell the computer what it needs to do. The JavaScript instructions are called functions.
FUNCTION: A function in JavaScript is like a factory. It might be a little factory with only one machine inside. Or it might contain many other little factories, each with many machines doing different jobs. In a real life clothes factory you might have reams of cloth and bobbins of thread going in and T-shirts and jeans coming out. Our JavaScript factory only processes data, it can't sew, drill a hole or melt metal. In our JavaScript factory data goes in and data comes out.
All this data stuff sounds a bit boring, but it is really very cool; we might have a function that tells a robot what to make for dinner. Let's say I invite you and your friend to my house. You like chicken legs best, I like sausages, your friend always wants what you want and my friend does not eat meat.
I haven't got time to go shopping, so the function needs to know what we have in the fridge to make decisions. Each ingredient has a different cooking time and we want everything to be served hot by the robot at the same time. We need to provide the function with the data about what we like, the function could 'talk' to the fridge, and the function could control the robot.
A function normally has a name, parentheses and braces. Like this:
function cookMeal() { /* STUFF INSIDE THE FUNCTION */ }
Note that /*...*/ and // stop code being read by the browser.
NAME: You can call a function just about whatever word you want. The example "cookMeal" is typical in joining two words together and giving the second one a capital letter at the beginning - but this is not necessary. It can't have a space in it, and it can't be a number on its own.
PARENTHESES: "Parentheses" or () are the letter box on the JavaScript function factory's door or a post box in the street for sending packets of information to the factory. Sometimes the postbox might be marked for example cookMeal(you, me, yourFriend, myFriend, fridge, dinnerTime), in which case you know what data you have to give it.
BRACES: "Braces" which look like this {} are the tinted windows of our factory. From inside the factory you can see out, but from the outside you can't see in.
THE LONG CODE EXAMPLE ABOVE
Our code begins with the word function, so we know that it is one! Then the name of the function sing - that's my own description of what the function is about. Then parentheses (). The parentheses are always there for a function. Sometimes they are empty, and sometimes they have something in. This one has a word in: (person). After this there is a brace like this { . This marks the start of the function sing(). It has a partner which marks the end of sing() like this }
function sing(person) { /* STUFF INSIDE THE FUNCTION */ }
So this function might have something to do with singing, and might need some data about a person. It has instructions inside to do something with that data.
Now, after the function sing(), near the end of the code is the line
var person="an old lady";
VARIABLE: The letters var stand for "variable". A variable is like an envelope. On the outside this envelope is marked "person". On the inside it contains a slip of paper with the information our function needs, some letters and spaces joined together like a piece of string (it's called a string) that make a phrase reading "an old lady". Our envelope could contain other kinds of things like numbers (called integers), instructions (called functions), lists (called arrays). Because this variable is written outside of all the braces {}, and because you can see out through the tinted windows when you are inside the braces, this variable can be seen from anywhere in the code. We call this a 'global variable'.
GLOBAL VARIABLE: person is a global variable, meaning that if you change its value from "an old lady" to "a young man", the person will keep being a young man until you decide to change it again and that any other function in the code can see that it's a young man. Press the F12 button or look at the Options settings to open the developer console of a browser and type "person" to see what this value is. Type person="a young man" to change it and then type "person" again to see that it has changed.
After this we have the line
sing(person);
This line is calling the function, as if it were calling a dog
"Come on sing, Come and get person!"
When the browser has loaded the JavaScript code an reached this line, it will start the function. I put the line at the end to make sure that the browser has all the information it needs to run it.
Functions define actions - the main function is about singing. It contains a variable called firstPart which applies to the singing about the person that applies to each of the verses of the song: "There was " + person + " who swallowed". If you type firstPart into the console, you won't get an answer because the variable is locked up in a function - the browser can't see inside the tinted windows of the braces.
CLOSURES: The closures are the smaller functions that are inside the big sing() function. The little factories inside the big factory. They each have their own braces which mean that the variables inside them can't be seen from the outside. That's why the names of the variables (creature and result) can be repeated in the closures but with different values. If you type these variable names in the console window, you won't get its value because it's hidden by two layers of tinted windows.
The closures all know what the sing() function's variable called firstPart is, because they can see out from their tinted windows.
After the closures come the lines
fly();
spider();
bird();
cat();
The sing() function will call each of these functions in the order they are given. Then the sing() function's work will be done.
Okay, talking with a 6-year old child, I would possibly use following associations.
Imagine - you are playing with your little brothers and sisters in the entire house, and you are moving around with your toys and brought some of them into your older brother's room. After a while your brother returned from the school and went to his room, and he locked inside it, so now you could not access toys left there anymore in a direct way. But you could knock the door and ask your brother for that toys. This is called toy's closure; your brother made it up for you, and he is now into outer scope.
Compare with a situation when a door was locked by draft and nobody inside (general function execution), and then some local fire occur and burn down the room (garbage collector:D), and then a new room was build and now you may leave another toys there (new function instance), but never get the same toys which were left in the first room instance.
For an advanced child I would put something like the following. It is not perfect, but it makes you feel about what it is:
function playingInBrothersRoom (withToys) {
// We closure toys which we played in the brother's room. When he come back and lock the door
// your brother is supposed to be into the outer [[scope]] object now. Thanks god you could communicate with him.
var closureToys = withToys || [],
returnToy, countIt, toy; // Just another closure helpers, for brother's inner use.
var brotherGivesToyBack = function (toy) {
// New request. There is not yet closureToys on brother's hand yet. Give him a time.
returnToy = null;
if (toy && closureToys.length > 0) { // If we ask for a specific toy, the brother is going to search for it.
for ( countIt = closureToys.length; countIt; countIt--) {
if (closureToys[countIt - 1] == toy) {
returnToy = 'Take your ' + closureToys.splice(countIt - 1, 1) + ', little boy!';
break;
}
}
returnToy = returnToy || 'Hey, I could not find any ' + toy + ' here. Look for it in another room.';
}
else if (closureToys.length > 0) { // Otherwise, just give back everything he has in the room.
returnToy = 'Behold! ' + closureToys.join(', ') + '.';
closureToys = [];
}
else {
returnToy = 'Hey, lil shrimp, I gave you everything!';
}
console.log(returnToy);
}
return brotherGivesToyBack;
}
// You are playing in the house, including the brother's room.
var toys = ['teddybear', 'car', 'jumpingrope'],
askBrotherForClosuredToy = playingInBrothersRoom(toys);
// The door is locked, and the brother came from the school. You could not cheat and take it out directly.
console.log(askBrotherForClosuredToy.closureToys); // Undefined
// But you could ask your brother politely, to give it back.
askBrotherForClosuredToy('teddybear'); // Hooray, here it is, teddybear
askBrotherForClosuredToy('ball'); // The brother would not be able to find it.
askBrotherForClosuredToy(); // The brother gives you all the rest
askBrotherForClosuredToy(); // Nothing left in there
As you can see, the toys left in the room are still accessible via the brother and no matter if the room is locked. Here is a jsbin to play around with it.
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.
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.
I'd simply point them to the Mozilla Closures page. It's the best, most concise and simple explanation of closure basics and practical usage that I've found. It is highly recommended to anyone learning JavaScript.
And yes, I'd even recommend it to a 6-year old -- if the 6-year old is learning about closures, then it's logical they're ready to comprehend the concise and simple explanation provided in the article.
I have a constructor function which registers an event handler:
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', function () {
alert(this.data);
});
}
// Mock transport object
var transport = {
on: function(event, callback) {
setTimeout(callback, 1000);
}
};
// called as
var obj = new MyConstructor('foo', transport);
However, I'm not able to access the data property of the created object inside the callback. It looks like this does not refer to the object that was created, but to another one.
I also tried to use an object method instead of an anonymous function:
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', this.alert);
}
MyConstructor.prototype.alert = function() {
alert(this.name);
};
but it exhibits the same problems.
How can I access the correct object?
What you should know about this
this (aka "the context") is a special keyword inside each function and its value only depends on how the function was called, not how/when/where it was defined. It is not affected by lexical scopes like other variables (except for arrow functions, see below). Here are some examples:
function foo() {
console.log(this);
}
// normal function call
foo(); // `this` will refer to `window`
// as object method
var obj = {bar: foo};
obj.bar(); // `this` will refer to `obj`
// as constructor function
new foo(); // `this` will refer to an object that inherits from `foo.prototype`
To learn more about this, have a look at the MDN documentation.
How to refer to the correct this
Use arrow functions
ECMAScript 6 introduced arrow functions, which can be thought of as lambda functions. They don't have their own this binding. Instead, this is looked up in scope just like a normal variable. That means you don't have to call .bind. That's not the only special behavior they have, please refer to the MDN documentation for more information.
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => alert(this.data));
}
Don't use this
You actually don't want to access this in particular, but the object it refers to. That's why an easy solution is to simply create a new variable that also refers to that object. The variable can have any name, but common ones are self and that.
function MyConstructor(data, transport) {
this.data = data;
var self = this;
transport.on('data', function() {
alert(self.data);
});
}
Since self is a normal variable, it obeys lexical scope rules and is accessible inside the callback. This also has the advantage that you can access the this value of the callback itself.
Explicitly set this of the callback - part 1
It might look like you have no control over the value of this because its value is set automatically, but that is actually not the case.
Every function has the method .bind [docs], which returns a new function with this bound to a value. The function has exactly the same behavior as the one you called .bind on, only that this was set by you. No matter how or when that function is called, this will always refer to the passed value.
function MyConstructor(data, transport) {
this.data = data;
var boundFunction = (function() { // parenthesis are not necessary
alert(this.data); // but might improve readability
}).bind(this); // <- here we are calling `.bind()`
transport.on('data', boundFunction);
}
In this case, we are binding the callback's this to the value of MyConstructor's this.
Note: When a binding context for jQuery, use jQuery.proxy [docs] instead. The reason to do this is so that you don't need to store the reference to the function when unbinding an event callback. jQuery handles that internally.
Set this of the callback - part 2
Some functions/methods which accept callbacks also accept a value to which the callback's this should refer to. This is basically the same as binding it yourself, but the function/method does it for you. Array#map [docs] is such a method. Its signature is:
array.map(callback[, thisArg])
The first argument is the callback and the second argument is the value this should refer to. Here is a contrived example:
var arr = [1, 2, 3];
var obj = {multiplier: 42};
var new_arr = arr.map(function(v) {
return v * this.multiplier;
}, obj); // <- here we are passing `obj` as second argument
Note: Whether or not you can pass a value for this is usually mentioned in the documentation of that function/method. For example, jQuery's $.ajax method [docs] describes an option called context:
This object will be made the context of all Ajax-related callbacks.
Common problem: Using object methods as callbacks/event handlers
Another common manifestation of this problem is when an object method is used as callback/event handler. Functions are first-class citizens in JavaScript and the term "method" is just a colloquial term for a function that is a value of an object property. But that function doesn't have a specific link to its "containing" object.
Consider the following example:
function Foo() {
this.data = 42,
document.body.onclick = this.method;
}
Foo.prototype.method = function() {
console.log(this.data);
};
The function this.method is assigned as click event handler, but if the document.body is clicked, the value logged will be undefined, because inside the event handler, this refers to the document.body, not the instance of Foo.
As already mentioned at the beginning, what this refers to depends on how the function is called, not how it is defined.
If the code was like the following, it might be more obvious that the function doesn't have an implicit reference to the object:
function method() {
console.log(this.data);
}
function Foo() {
this.data = 42,
document.body.onclick = this.method;
}
Foo.prototype.method = method;
The solution is the same as mentioned above: If available, use .bind to explicitly bind this to a specific value
document.body.onclick = this.method.bind(this);
or explicitly call the function as a "method" of the object, by using an anonymous function as callback / event handler and assign the object (this) to another variable:
var self = this;
document.body.onclick = function() {
self.method();
};
or use an arrow function:
document.body.onclick = () => this.method();
Here are several ways to access the parent context inside a child context -
You can use the bind() function.
Store a reference to context/this inside another variable (see the below example).
Use ES6 Arrow functions.
Alter the code, function design, and architecture - for this you should have command over design patterns in JavaScript.
1. Use the bind() function
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', ( function () {
alert(this.data);
}).bind(this) );
}
// Mock transport object
var transport = {
on: function(event, callback) {
setTimeout(callback, 1000);
}
};
// called as
var obj = new MyConstructor('foo', transport);
If you are using Underscore.js - http://underscorejs.org/#bind
transport.on('data', _.bind(function () {
alert(this.data);
}, this));
2. Store a reference to context/this inside another variable
function MyConstructor(data, transport) {
var self = this;
this.data = data;
transport.on('data', function() {
alert(self.data);
});
}
3. Arrow function
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => {
alert(this.data);
});
}
It's all in the "magic" syntax of calling a method:
object.property();
When you get the property from the object and call it in one go, the object will be the context for the method. If you call the same method, but in separate steps, the context is the global scope (window) instead:
var f = object.property;
f();
When you get the reference of a method, it's no longer attached to the object. It's just a reference to a plain function. The same happens when you get the reference to use as a callback:
this.saveNextLevelData(this.setAll);
That's where you would bind the context to the function:
this.saveNextLevelData(this.setAll.bind(this));
If you are using jQuery you should use the $.proxy method instead, as bind is not supported in all browsers:
this.saveNextLevelData($.proxy(this.setAll, this));
You should know about "this" Keyword.
As per my view you can implement "this" in three ways
(Self|Arrow function|Bind Method)
A function's this keyword behaves a little differently in JavaScript compared to other languages.
It also has some differences between strict mode and non-strict mode.
In most cases, the value of this is determined by how a function is called.
It can't be set by assignment during execution, and it may be different each time the function is called.
ES5 introduced the bind() method to set the value of a function's this regardless of how it's called,
And ES2015 introduced arrow functions that don't provide their own this binding (it retains this value of the enclosing lexical context).
Method1: Self - Self is being used to maintain a reference to the original this even as the context is changing. It's a technique often used in event handlers (especially in closures).
Reference: this
function MyConstructor(data, transport) {
this.data = data;
var self = this;
transport.on('data', function () {
alert(self.data);
});
}
Method2: Arrow function - An arrow function expression is a syntactically compact alternative to a regular function expression, although without its own bindings to the this, arguments, super, or new.target keywords.
Arrow function expressions are ill-suited as methods, and they cannot be used as constructors.
Reference: Arrow function expressions
function MyConstructor(data, transport) {
this.data = data;
transport.on('data',()=> {
alert(this.data);
});
}
Method 3: Bind - The bind() method creates a new function that, when called, has its this keyword set to the provided value with a given sequence of arguments preceding any provided when the new function is called.
Reference: Function.prototype.bind()
function MyConstructor(data, transport) {
this.data = data;
transport.on('data',(function() {
alert(this.data);
}).bind(this);
The trouble with "context"
The term "context" is sometimes used to refer to the object referenced by this. Its use is inappropriate, because it doesn't fit either semantically or technically with ECMAScript's this.
"Context" means the circumstances surrounding something that adds meaning, or some preceding and following information that gives extra meaning. The term "context" is used in ECMAScript to refer to execution context, which is all the parameters, scope, and this within the scope of some executing code.
This is shown in ECMA-262 section 10.4.2:
Set the ThisBinding to the same value as the ThisBinding of the
calling execution context
Which clearly indicates that this is part of an execution context.
An execution context provides the surrounding information that adds meaning to the code that is being executed. It includes much more information than just the thisBinding.
The value of this isn't "context". It's just one part of an execution context. It's essentially a local variable that can be set by the call to any object and in strict mode, to any value at all.
First, you need to have a clear understanding of scope and behaviour of the this keyword in the context of scope.
this & scope:
There are two types of scope in JavaScript. They are:
Global Scope
Function Scope
In short, global scope refers to the window object. Variables declared in a global scope are accessible from anywhere.
On the other hand, function scope resides inside of a function. A variable declared inside a function cannot be accessed from the outside world normally.
The this keyword in the global scope refers to the window object. this inside a function also refers to the window object. So this will always refer to the window until we find a way to manipulate this to indicate a context of our own choosing.
--------------------------------------------------------------------------------
- -
- Global Scope -
- (globally "this" refers to window object) -
- -
- function outer_function(callback){ -
- -
- // Outer function scope -
- // Inside the outer function, the "this" keyword -
- // refers to window object -
- callback() // "this" inside callback also refers to the window object -
- } -
- -
- function callback_function(){ -
- -
- // Function to be passed as callback -
- -
- // Here "THIS" refers to the window object also -
- } -
- -
- outer_function(callback_function) -
- // Invoke with callback -
- -
--------------------------------------------------------------------------------
Different ways to manipulate this inside callback functions:
Here I have a constructor function called Person. It has a property called name and four method called sayNameVersion1, sayNameVersion2, sayNameVersion3, and sayNameVersion4. All four of them has one specific task. Accept a callback and invoke it. The callback has a specific task which is to log the name property of an instance of Person constructor function.
function Person(name){
this.name = name
this.sayNameVersion1 = function(callback){
callback.bind(this)()
}
this.sayNameVersion2 = function(callback){
callback()
}
this.sayNameVersion3 = function(callback){
callback.call(this)
}
this.sayNameVersion4 = function(callback){
callback.apply(this)
}
}
function niceCallback(){
// Function to be used as callback
var parentObject = this
console.log(parentObject)
}
Now let's create an instance from person constructor and invoke different versions of sayNameVersionX (X refers to 1,2,3,4) method with niceCallback to see how many ways we can manipulate the this inside callback to refer to the person instance.
var p1 = new Person('zami') // Create an instance of Person constructor
bind:
What bind do is to create a new function with the this keyword set to the provided value.
sayNameVersion1 and sayNameVersion2 use bind to manipulate this of the callback function.
this.sayNameVersion1 = function(callback){
callback.bind(this)()
}
this.sayNameVersion2 = function(callback){
callback()
}
The first one binds this with a callback inside the method itself. And for the second one, the callback is passed with the object bound to it.
p1.sayNameVersion1(niceCallback) // pass simply the callback and bind happens inside the sayNameVersion1 method
p1.sayNameVersion2(niceCallback.bind(p1)) // uses bind before passing callback
call:
The first argument of the call method is used as this inside the function that is invoked with call attached to it.
sayNameVersion3 uses call to manipulate the this to refer to the person object that we created, instead of the window object.
this.sayNameVersion3 = function(callback){
callback.call(this)
}
And it is called like the following:
p1.sayNameVersion3(niceCallback)
apply:
Similar to call, the first argument of apply refers to the object that will be indicated by the this keyword.
sayNameVersion4 uses apply to manipulate this to refer to a person object
this.sayNameVersion4 = function(callback){
callback.apply(this)
}
And it is called like the following. Simply the callback is passed,
p1.sayNameVersion4(niceCallback)
We can not bind this to setTimeout(), as it always executes with the global object (Window). If you want to access the this context in the callback function then by using bind() to the callback function, we can achieve it as:
setTimeout(function(){
this.methodName();
}.bind(this), 2000);
The question revolves around how the this keyword behaves in JavaScript. this behaves differently as below,
The value of this is usually determined by a function execution context.
In the global scope, this refers to the global object (the window object).
If strict mode is enabled for any function then the value of this will be undefined as in strict mode, global object refers to undefined in place of the window object.
The object that is standing before the dot is what the this keyword will be bound to.
We can set the value of this explicitly with call(), bind(), and apply()
When the new keyword is used (a constructor), this is bound to the new object being created.
Arrow functions don’t bind this — instead, this is bound lexically (i.e., based on the original context)
As most of the answers suggest, we can use the arrow function or bind() Method or Self var. I would quote a point about lambdas (arrow function) from Google JavaScript Style Guide
Prefer using arrow functions over f.bind(this), and especially over
goog.bind(f, this). Avoid writing const self = this. Arrow functions
are particularly useful for callbacks, which sometimes pass unexpectedly
additional arguments.
Google clearly recommends using lambdas rather than bind or const self = this
So the best solution would be to use lambdas as below,
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => {
alert(this.data);
});
}
References:
https://medium.com/tech-tajawal/javascript-this-4-rules-7354abdb274c
arrow-functions-vs-bind
Currently there is another approach possible if classes are used in code.
With support of class fields, it's possible to make it the following way:
class someView {
onSomeInputKeyUp = (event) => {
console.log(this); // This refers to the correct value
// ....
someInitMethod() {
//...
someInput.addEventListener('input', this.onSomeInputKeyUp)
For sure under the hood it's all the old good arrow function that binds context, but in this form it looks much more clear that explicit binding.
Since it's a Stage 3 Proposal, you will need Babel and appropriate Babel plugin to process it as for now (08/2018).
I was facing a problem with Ngx line chart xAxisTickFormatting function which was called from HTML like this: [xAxisTickFormatting]="xFormat".
I was unable to access my component's variable from the function declared. This solution helped me to resolve the issue to find the correct this.
Instead of using the function like this:
xFormat (value): string {
return value.toString() + this.oneComponentVariable; //gives wrong result
}
Use this:
xFormat = (value) => {
// console.log(this);
// now you have access to your component variables
return value + this.oneComponentVariable
}
Another approach, which is the standard way since DOM2 to bind this within the event listener, that let you always remove the listener (among other benefits), is the handleEvent(evt) method from the EventListener interface:
var obj = {
handleEvent(e) {
// always true
console.log(this === obj);
}
};
document.body.addEventListener('click', obj);
Detailed information about using handleEvent can be found here: DOM handleEvent: a cross-platform standard since year 2000
Some other people have touched on how to use the .bind() method, but specifically here is how you can use it with .then() if anyone is having trouble getting them to work together:
someFunction()
.then(function(response) {
//'this' wasn't accessible here before but now it is
}.bind(this))
As mentioned in the comments, an alternative would be to use an arrow function that doesn't have its own 'this' value
someFunction()
.then((response)=>{
//'this' was always accessible here
})
this in JavaScript:
The value of this in JavaScript is 100% determined by how a function is called, and not how it is defined. We can relatively easily find the value of this by the 'left of the dot rule':
When the function is created using the function keyword the value of this is the object left of the dot of the function which is called
If there is no object left of the dot then the value of this inside a function is often the global object (global in Node.js and window in a browser). I wouldn't recommend using the this keyword here because it is less explicit than using something like window!
There exist certain constructs like arrow functions and functions created using the Function.prototype.bind() a function that can fix the value of this. These are exceptions of the rule, but they are really helpful to fix the value of this.
Example in Node.js
module.exports.data = 'module data';
// This outside a function in node refers to module.exports object
console.log(this);
const obj1 = {
data: "obj1 data",
met1: function () {
console.log(this.data);
},
met2: () => {
console.log(this.data);
},
};
const obj2 = {
data: "obj2 data",
test1: function () {
console.log(this.data);
},
test2: function () {
console.log(this.data);
}.bind(obj1),
test3: obj1.met1,
test4: obj1.met2,
};
obj2.test1();
obj2.test2();
obj2.test3();
obj2.test4();
obj1.met1.call(obj2);
Output:
Let me walk you through the outputs one by one (ignoring the first log starting from the second):
this is obj2 because of the left of the dot rule, we can see how test1 is called obj2.test1();. obj2 is left of the dot and thus the this value.
Even though obj2 is left of the dot, test2 is bound to obj1 via the bind() method. The this value is obj1.
obj2 is left of the dot from the function which is called: obj2.test3(). Therefore obj2 will be the value of this.
In this case: obj2.test4() obj2 is left of the dot. However, arrow functions don't have their own this binding. Therefore it will bind to the this value of the outer scope which is the module.exports an object which was logged in the beginning.
We can also specify the value of this by using the call function. Here we can pass in the desired this value as an argument, which is obj2 in this case.