Javascript Function Scoped For Loops - javascript

Here's an example of a situation where a simple JS loop does not behave as expected, because of the loop variable not being in a separate scope.
The solution often presented is to construct an unpleasant-looking bit of loop code that looks like this:
for (var i in obj) {
(function() {
... obj[i] ...
// this new shadowed i here is now no longer getting changed by for loop
})(i);
}
My question is, could this be improved upon? Could I use this:
Object.prototype.each = function (f) {
for (var i in this) {
f(i,this[i]);
}
};
// leading to this somewhat more straightforward invocation
obj.each(
function(i,v) {
... v ...
// alternatively, v is identical to
... obj[i] ...
}
);
when I ascertain that I need a "scoped loop"? It is somewhat cleaner looking and should have similar performance to the regular for-loop (since it uses it the same way).
Update: It seems that doing things with Object.prototype is a huge no-no because it breaks pretty much everything.
Here is a less intrusive implementation:
function each (obj,f) {
for (var i in obj) {
f(i,obj[i]);
}
}
The invocation changes very slightly to
each(obj,
function(i,v) {
... v ...
}
);
So I guess I've answered my own question, if jQuery does it this way, can't really go wrong. Any issues I've overlooked though would warrant an answer.


Your answer pretty much covers it, but I think a change in your original loop is worth noting as it makes it reasonable to use a normal for loop when the each() function isn't handy, for whatever reason.
Update: Changed to use an example that's similar to the example referenced by the question to compare the different approaches. The example had to be adjusted because the each() function requires a populated array to iterate over.
Assuming the following setup:
var vals = ['a', 'b', 'c', 'd'],
max = vals.length,
closures = [],
i;
Using the example from the question, the original loop ends up creating 2n functions (where n is the number of iterations) because two functions are created during each iteration:
for (i = 0; i < max; i++) {
closures[i] = (function(idx, val) { // 1st - factoryFn - captures the values as arguments
return function() { // 2nd - alertFn - uses the arguments instead
alert(idx + ' -> ' + val); // of the variables
};
})(i, vals[i]);
}
This can be reduced to creating only n + 1 functions by creating the factory function once, before the loop is started, and then reusing it:
var factoryFn = function(idx, val) {
return function() {
alert(idx + ' -> ' + val);
};
};
for (i = 0; i < max; i++) {
closures[i] = factoryFn(i, vals[i]);
}
This is nearly equivalent to how the each() function might be used in this situation, which would also result in a total of n + 1 functions created. The factory function is created once and passed immediately as an argument to each().
each(vals, function(idx, val) {
closures[idx] = function() {
alert(idx + ' -> ' + val);
};
});
FWIW, I think a benefit to using each() is the code is a bit shorter and creating the factory function right as it's passed into the each() function clearly illustrates this is its only use. A benefit of the for loop version, IMO, is the code that does the loop is right there so it's nature and behavior is completely transparent while the each() function might be defined in a different file, written by someone else, etc.


Global Scope
When something is global means that it is accessible from anywhere in your code. Take this for example:
var monkey = "Gorilla";
function greetVisitor () {
return alert("Hello dear blog reader!");
}
If that code was being run in a web browser, the function scope would be window, thus making it
available to everything running in that web browser window.
Local Scope
As opposed to the global scope, the local scope is when something is just defined and accessible in a
certain part of the code, like a function. For instance;
function talkDirty () {
var saying = "Oh, you little VB lover, you";
return alert(saying);
}
alert(saying); // Throws an error
If you take a look at the code above, the variable saying is only available within the talkDirty
function. Outside of it it isn’t defined at all. Note of caution: if you were to declare saying without
the var keyword preceding it, it would automatically become a global variable.
What this also means is that if you have nested functions, the inner function will have access to the
containing functions variables and functions:
function saveName (firstName) {
function capitalizeName () {
return firstName.toUpperCase();
}
var capitalized = capitalizeName();
return capitalized;
}
alert(saveName("Robert")); // Returns "ROBERT"
As you just saw, the inner function capitalizeName didn’t need any parameter sent in, but had complete
access to the parameter firstName in the outer saveName function. For clarity, let’s take another
example:
function siblings () {
var siblings = ["John", "Liza", "Peter"];
function siblingCount () {
var siblingsLength = siblings.length;
return siblingsLength;
}
function joinSiblingNames () {
return "I have " + siblingCount() + " siblings:\n\n" + siblings.join("\n");
}
return joinSiblingNames();
}
alert(siblings()); // Outputs "I have 3 siblings: John Liza Peter"
As you just saw, both inner functions have access to the siblings array in the containing function, and
each inner function have access to the other inner functions on the same level (in this case,
joinSiblingNames can access siblingCount). However, the variable siblingsLength in the siblingCount is
only available within that function, i.e. that scope.

Related

why return a function inside another function in javascript? is there any advantages? [duplicate]

I'm trying my hardest to wrap my head around JavaScript closures.
I get that by returning an inner function, it will have access to any variable defined in its immediate parent.
Where would this be useful to me? Perhaps I haven't quite got my head around it yet. Most of the examples I have seen online don't provide any real world code, just vague examples.
Can someone show me a real world use of a closure?
Is this one, for example?
var warnUser = function (msg) {
var calledCount = 0;
return function() {
calledCount++;
alert(msg + '\nYou have been warned ' + calledCount + ' times.');
};
};
var warnForTamper = warnUser('You can not tamper with our HTML.');
warnForTamper();
warnForTamper();

Suppose, you want to count the number of times user clicked a button on a webpage.
For this, you are triggering a function on onclick event of button to update the count of the variable
<button onclick="updateClickCount()">click me</button>
Now there could be many approaches like:
You could use a global variable, and a function to increase the counter:
var counter = 0;
function updateClickCount() {
++counter;
// Do something with counter
}
But, the pitfall is that any script on the page can change the counter, without calling updateClickCount().
Now, you might be thinking of declaring the variable inside the function:
function updateClickCount() {
var counter = 0;
++counter;
// Do something with counter
}
But, hey! Every time updateClickCount() function is called, the counter is set to 1 again.
Thinking about nested functions?
Nested functions have access to the scope "above" them.
In this example, the inner function updateClickCount() has access to the counter variable in the parent function countWrapper():
function countWrapper() {
var counter = 0;
function updateClickCount() {
++counter;
// Do something with counter
}
updateClickCount();
return counter;
}
This could have solved the counter dilemma, if you could reach the updateClickCount() function from the outside and you also need to find a way to execute counter = 0 only once not everytime.
Closure to the rescue! (self-invoking function):
var updateClickCount = (function(){
var counter = 0;
return function(){
++counter;
// Do something with counter
}
})();
The self-invoking function only runs once. It sets the counter to zero (0), and returns a function expression.
This way updateClickCount becomes a function. The "wonderful" part is that it can access the counter in the parent scope.
This is called a JavaScript closure. It makes it possible for a function to have "private" variables.
The counter is protected by the scope of the anonymous function, and can only be changed using the updateClickCount() function!
A more lively example on closures
<script>
var updateClickCount = (function(){
var counter = 0;
return function(){
++counter;
document.getElementById("spnCount").innerHTML = counter;
}
})();
</script>
<html>
<button onclick="updateClickCount()">click me</button>
<div> you've clicked
<span id="spnCount"> 0 </span> times!
</div>
</html>
Reference: JavaScript Closures

I've used closures to do things like:
a = (function () {
var privatefunction = function () {
alert('hello');
}
return {
publicfunction : function () {
privatefunction();
}
}
})();
As you can see there, a is now an object, with a method publicfunction ( a.publicfunction() ) which calls privatefunction, which only exists inside the closure. You can not call privatefunction directly (i.e. a.privatefunction() ), just publicfunction().
It's a minimal example, but maybe you can see uses to it? We used this to enforce public/private methods.

The example you give is an excellent one. Closures are an abstraction mechanism that allow you to separate concerns very cleanly. Your example is a case of separating instrumentation (counting calls) from semantics (an error-reporting API). Other uses include:
Passing parameterised behaviour into an algorithm (classic higher-order programming):
function proximity_sort(arr, midpoint) {
arr.sort(function(a, b) { a -= midpoint; b -= midpoint; return a*a - b*b; });
}
Simulating object oriented programming:
function counter() {
var a = 0;
return {
inc: function() { ++a; },
dec: function() { --a; },
get: function() { return a; },
reset: function() { a = 0; }
}
}
Implementing exotic flow control, such as jQuery's Event handling and AJAX APIs.

JavaScript closures can be used to implement throttle and debounce functionality in your application.
Throttling
Throttling puts a limit on as a maximum number of times a function can be called over time. As in "execute this function at most once every 100 milliseconds."
Code:
const throttle = (func, limit) => {
let isThrottling
return function() {
const args = arguments
const context = this
if (!isThrottling) {
func.apply(context, args)
isThrottling = true
setTimeout(() => isThrottling = false, limit)
}
}
}
Debouncing
Debouncing puts a limit on a function not be called again until a certain amount of time has passed without it being called. As in "execute this function only if 100 milliseconds have passed without it being called."
Code:
const debounce = (func, delay) => {
let debouncing
return function() {
const context = this
const args = arguments
clearTimeout(debouncing)
debouncing = setTimeout(() => func.apply(context, args), delay)
}
}
As you can see closures helped in implementing two beautiful features which every web application should have to provide smooth UI experience functionality.

Yes, that is a good example of a useful closure. The call to warnUser creates the calledCount variable in its scope and returns an anonymous function which is stored in the warnForTamper variable. Because there is still a closure making use of the calledCount variable, it isn't deleted upon the function's exit, so each call to the warnForTamper() will increase the scoped variable and alert the value.
The most common issue I see on Stack Overflow is where someone wants to "delay" use of a variable that is increased upon each loop, but because the variable is scoped then each reference to the variable would be after the loop has ended, resulting in the end state of the variable:
for (var i = 0; i < someVar.length; i++)
window.setTimeout(function () {
alert("Value of i was "+i+" when this timer was set" )
}, 10000);
This would result in every alert showing the same value of i, the value it was increased to when the loop ended. The solution is to create a new closure, a separate scope for the variable. This can be done using an instantly executed anonymous function, which receives the variable and stores its state as an argument:
for (var i = 0; i < someVar.length; i++)
(function (i) {
window.setTimeout(function () {
alert("Value of i was " + i + " when this timer was set")
}, 10000);
})(i);

In the JavaScript (or any ECMAScript) language, in particular, closures are useful in hiding the implementation of functionality while still revealing the interface.
For example, imagine you are writing a class of date utility methods and you want to allow users to look up weekday names by index, but you don't want them to be able to modify the array of names you use under the hood.
var dateUtil = {
weekdayShort: (function() {
var days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'];
return function(x) {
if ((x != parseInt(x)) || (x < 1) || (x > 7)) {
throw new Error("invalid weekday number");
}
return days[x - 1];
};
}())
};
Note that the days array could simply be stored as a property of the dateUtil object, but then it would be visible to users of the script and they could even change it if they wanted, without even needing your source code. However, since it's enclosed by the anonymous function which returns the date lookup function it is only accessible by the lookup function so it is now tamperproof.

There is a section on Practical Closures at the Mozilla Developer Network.

If you're comfortable with the concept of instantiating a class in the object-oriented sense (i.e. to create an object of that class) then you're close to understanding closures.
Think of it this way: when you instantiate two Person objects you know that the class member variable "Name" is not shared between instances; each object has its own 'copy'. Similarly, when you create a closure, the free variable ('calledCount' in your example above) is bound to the 'instance' of the function.
I think your conceptual leap is slightly hampered by the fact that every function/closure returned by the warnUser function (aside: that's a higher-order function) closure binds 'calledCount' with the same initial value (0), whereas often when creating closures it is more useful to pass different initializers into the higher-order function, much like passing different values to the constructor of a class.
So, suppose when 'calledCount' reaches a certain value you want to end the user's session; you might want different values for that depending on whether the request comes in from the local network or the big bad internet (yes, it's a contrived example). To achieve this, you could pass different initial values for calledCount into warnUser (i.e. -3, or 0?).
Part of the problem with the literature is the nomenclature used to describe them ("lexical scope", "free variables"). Don't let it fool you, closures are more simple than would appear... prima facie ;-)

Here, I have a greeting that I want to say several times. If I create a closure, I can simply call that function to record the greeting. If I don't create the closure, I have to pass my name in every single time.
Without a closure (https://jsfiddle.net/lukeschlangen/pw61qrow/3/):
function greeting(firstName, lastName) {
var message = "Hello " + firstName + " " + lastName + "!";
console.log(message);
}
greeting("Billy", "Bob");
greeting("Billy", "Bob");
greeting("Billy", "Bob");
greeting("Luke", "Schlangen");
greeting("Luke", "Schlangen");
greeting("Luke", "Schlangen");
With a closure (https://jsfiddle.net/lukeschlangen/Lb5cfve9/3/):
function greeting(firstName, lastName) {
var message = "Hello " + firstName + " " + lastName + "!";
return function() {
console.log(message);
}
}
var greetingBilly = greeting("Billy", "Bob");
var greetingLuke = greeting("Luke", "Schlangen");
greetingBilly();
greetingBilly();
greetingBilly();
greetingLuke();
greetingLuke();
greetingLuke();

Another common use for closures is to bind this in a method to a specific object, allowing it to be called elsewhere (such as as an event handler).
function bind(obj, method) {
if (typeof method == 'string') {
method = obj[method];
}
return function () {
method.apply(obj, arguments);
}
}
...
document.body.addEventListener('mousemove', bind(watcher, 'follow'), true);
Whenever a mousemove event fires, watcher.follow(evt) is called.
Closures are also an essential part of higher-order functions, allowing the very common pattern of rewriting multiple similar functions as a single higher order function by parameterizing the dissimilar portions. As an abstract example,
foo_a = function (...) {A a B}
foo_b = function (...) {A b B}
foo_c = function (...) {A c B}
becomes
fooer = function (x) {
return function (...) {A x B}
}
where A and B aren't syntactical units but source code strings (not string literals).
See "Streamlining my javascript with a function" for a concrete example.

Here I have one simple example of the closure concept which we can use for in our E-commerce site or many others as well.
I am adding my JSFiddle link with the example. It contains a small product list of three items and one cart counter.
JSFiddle
// Counter closure implemented function;
var CartCouter = function(){
var counter = 0;
function changeCounter(val){
counter += val
}
return {
increment: function(){
changeCounter(1);
},
decrement: function(){
changeCounter(-1);
},
value: function(){
return counter;
}
}
}
var cartCount = CartCouter();
function updateCart() {
document.getElementById('cartcount').innerHTML = cartCount.value();
}
var productlist = document.getElementsByClassName('item');
for(var i = 0; i< productlist.length; i++){
productlist[i].addEventListener('click', function(){
if(this.className.indexOf('selected') < 0){
this.className += " selected";
cartCount.increment();
updateCart();
}
else{
this.className = this.className.replace("selected", "");
cartCount.decrement();
updateCart();
}
})
}
.productslist{
padding: 10px;
}
ul li{
display: inline-block;
padding: 5px;
border: 1px solid #DDD;
text-align: center;
width: 25%;
cursor: pointer;
}
.selected{
background-color: #7CFEF0;
color: #333;
}
.cartdiv{
position: relative;
float: right;
padding: 5px;
box-sizing: border-box;
border: 1px solid #F1F1F1;
}
<div>
<h3>
Practical use of a JavaScript closure concept/private variable.
</h3>
<div class="cartdiv">
<span id="cartcount">0</span>
</div>
<div class="productslist">
<ul>
<li class="item">Product 1</li>
<li class="item">Product 2</li>
<li class="item">Product 3</li>
</ul>
</div>
</div>

Use of Closures:
Closures are one of the most powerful features of JavaScript. JavaScript allows for the nesting of functions and grants the inner function full access to all the variables and functions defined inside the outer function (and all other variables and functions that the outer function has access to). However, the outer function does not have access to the variables and functions defined inside the inner function.
This provides a sort of security for the variables of the inner function. Also, since the inner function has access to the scope of the outer function, the variables and functions defined in the outer function will live longer than the outer function itself, if the inner function manages to survive beyond the life of the outer function. A closure is created when the inner function is somehow made available to any scope outside the outer function.
Example:
<script>
var createPet = function(name) {
var sex;
return {
setName: function(newName) {
name = newName;
},
getName: function() {
return name;
},
getSex: function() {
return sex;
},
setSex: function(newSex) {
if(typeof newSex == "string" && (newSex.toLowerCase() == "male" || newSex.toLowerCase() == "female")) {
sex = newSex;
}
}
}
}
var pet = createPet("Vivie");
console.log(pet.getName()); // Vivie
console.log(pet.setName("Oliver"));
console.log(pet.setSex("male"));
console.log(pet.getSex()); // male
console.log(pet.getName()); // Oliver
</script>
In the code above, the name variable of the outer function is accessible to the inner functions, and there is no other way to access the inner variables except through the inner functions. The inner variables of the inner function act as safe stores for the inner functions. They hold "persistent", yet secure, data for the inner functions to work with. The functions do not even have to be assigned to a variable, or have a name.
read here for detail.

Explaining the practical use for a closure in JavaScript
When we create a function inside another function, we are creating a closure. Closures are powerful because they are capable of reading and manipulating the data of its outer functions. Whenever a function is invoked, a new scope is created for that call. The local variable declared inside the function belong to that scope and they can only be accessed from that function. When the function has finished the execution, the scope is usually destroyed.
A simple example of such function is this:
function buildName(name) {
const greeting = "Hello, " + name;
return greeting;
}
In above example, the function buildName() declares a local variable greeting and returns it. Every function call creates a new scope with a new local variable. After the function is done executing, we have no way to refer to that scope again, so it’s garbage collected.
But how about when we have a link to that scope?
Let’s look at the next function:
function buildName(name) {
const greeting = "Hello, " + name + " Welcome ";
const sayName = function() {
console.log(greeting);
};
return sayName;
}
const sayMyName = buildName("Mandeep");
sayMyName(); // Hello, Mandeep Welcome
The function sayName() from this example is a closure. The sayName() function has its own local scope (with variable welcome) and has also access to the outer (enclosing) function’s scope. In this case, the variable greeting from buildName().
After the execution of buildName is done, the scope is not destroyed in this case. The sayMyName() function still has access to it, so it won’t be garbage collected. However, there is no other way of accessing data from the outer scope except the closure. The closure serves as the gateway between the global context and the outer scope.

The JavaScript module pattern uses closures. Its nice pattern allows you to have something alike "public" and "private" variables.
var myNamespace = (function () {
var myPrivateVar, myPrivateMethod;
// A private counter variable
myPrivateVar = 0;
// A private function which logs any arguments
myPrivateMethod = function(foo) {
console.log(foo);
};
return {
// A public variable
myPublicVar: "foo",
// A public function utilizing privates
myPublicFunction: function(bar) {
// Increment our private counter
myPrivateVar++;
// Call our private method using bar
myPrivateMethod(bar);
}
};
})();

I like Mozilla's function factory example.
function makeAdder(x) {
return function(y) {
return x + y;
};
}
var addFive = makeAdder(5);
console.assert(addFive(2) === 7);
console.assert(addFive(-5) === 0);

This thread has helped me immensely in gaining a better understanding of how closures work.
I've since done some experimentation of my own and came up with this fairly simple code which may help some other people see how closures can be used in a practical way and how to use the closure at different levels to maintain variables similar to static and/or global variables without risk of them getting overwritten or confused with global variables.
This keeps track of button clicks, both at a local level for each individual button and a global level, counting every button click, contributing towards a single figure. Note I haven't used any global variables to do this, which is kind of the point of the exercise - having a handler that can be applied to any button that also contributes to something globally.
Please experts, do let me know if I've committed any bad practices here! I'm still learning this stuff myself.
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Closures on button presses</title>
<script type="text/javascript">
window.addEventListener("load" , function () {
/*
Grab the function from the first closure,
and assign to a temporary variable
this will set the totalButtonCount variable
that is used to count the total of all button clicks
*/
var buttonHandler = buttonsCount();
/*
Using the result from the first closure (a function is returned)
assign and run the sub closure that carries the
individual variable for button count and assign to the click handlers
*/
document.getElementById("button1").addEventListener("click" , buttonHandler() );
document.getElementById("button2").addEventListener("click" , buttonHandler() );
document.getElementById("button3").addEventListener("click" , buttonHandler() );
// Now that buttonHandler has served its purpose it can be deleted if needs be
buttonHandler = null;
});
function buttonsCount() {
/*
First closure level
- totalButtonCount acts as a sort of global counter to count any button presses
*/
var totalButtonCount = 0;
return function () {
// Second closure level
var myButtonCount = 0;
return function (event) {
// Actual function that is called on the button click
event.preventDefault();
/*
Increment the button counts.
myButtonCount only exists in the scope that is
applied to each event handler and therefore acts
to count each button individually, whereas because
of the first closure totalButtonCount exists at
the scope just outside, it maintains a sort
of static or global variable state
*/
totalButtonCount++;
myButtonCount++;
/*
Do something with the values ... fairly pointless
but it shows that each button contributes to both
its own variable and the outer variable in the
first closure
*/
console.log("Total button clicks: "+totalButtonCount);
console.log("This button count: "+myButtonCount);
}
}
}
</script>
</head>
<body>
Button 1
Button 2
Button 3
</body>
</html>

There are various use cases of closures.Here, I am going to explain most important usage of Closure concept.
Closure can be used to create private methods and variables just like an object-oriented language like java, c++ and so on. Once you implemented private methods and variables, your variables defined inside a function won't be accessible by window object. This helps in data hiding and data security.
const privateClass = () => {
let name = "sundar";
function setName(changeName) {
name = changeName;
}
function getName() {
return name;
}
return {
setName: setName,
getName: getName,
};
};
let javaLikeObject = privateClass(); \\ similar to new Class() in OOPS.
console.log(javaLikeObject.getName()); \\this will give sundar
javaLikeObject.setName("suresh");
console.log(javaLikeObject.getName()); \\this will give suresh
Another real-life example of closure :
Create index.html:
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Program with Javascript</title>
</head>
<body>
<p id="first"></p>
<p id="second"></p>
<button onclick="applyingConcepts()">Click</button>
<script src="./index.js"></script>
</body>
</html>
2)In index.js:
let count = 0;
return () => {
document.getElementById("first").innerHTML = count++;
};
})();
In this example, when you click a button, then your count will be updated on p#id.
Note: You might be wondering what's special in this code. When you inspect, you will notice that you can't change the value of count using the window object. This means you have declared private variable count so this prevents your states from being spoiled by the client.

I wrote an article a while back about how closures can be used to simplify event-handling code. It compares ASP.NET event handling to client-side jQuery.
http://www.hackification.com/2009/02/20/closures-simplify-event-handling-code/

Much of the code we write in front-end JavaScript is event-based — we define some behavior, then attach it to an event that is triggered by the user (such as a click or a keypress). Our code is generally attached as a callback: a single function which is executed in response to the event.
size12, size14, and size16 are now functions which will resize the body text to 12, 14, and 16 pixels, respectively. We can attach them to buttons (in this case links) as follows:
function makeSizer(size) {
return function() {
document.body.style.fontSize = size + 'px';
};
}
var size12 = makeSizer(12);
var size14 = makeSizer(14);
var size16 = makeSizer(16);
document.getElementById('size-12').onclick = size12;
document.getElementById('size-14').onclick = size14;
document.getElementById('size-16').onclick = size16;
Fiddle

Closures are a useful way to create generators, a sequence incremented on-demand:
var foobar = function(i){var count = count || i; return function(){return ++count;}}
baz = foobar(1);
console.log("first call: " + baz()); //2
console.log("second call: " + baz()); //3
The differences are summarized as follows:
Anonymous functions Defined functions
Cannot be used as a method Can be used as a method of an object
Exists only in the scope in which it is defined Exists within the object it is defined in
Can only be called in the scope in which it is defined Can be called at any point in the code
Can be reassigned a new value or deleted Cannot be deleted or changed
References
AS3 Fundamentals: Functions

I'm trying to learn closures and I think the example that I have created is a practical use case. You can run a snippet and see the result in the console.
We have two separate users who have separate data. Each of them can see the actual state and update it.
function createUserWarningData(user) {
const data = {
name: user,
numberOfWarnings: 0,
};
function addWarning() {
data.numberOfWarnings = data.numberOfWarnings + 1;
}
function getUserData() {
console.log(data);
return data;
}
return {
getUserData: getUserData,
addWarning: addWarning,
};
}
const user1 = createUserWarningData("Thomas");
const user2 = createUserWarningData("Alex");
//USER 1
user1.getUserData(); // Returning data user object
user1.addWarning(); // Add one warning to specific user
user1.getUserData(); // Returning data user object
//USER2
user2.getUserData(); // Returning data user object
user2.addWarning(); // Add one warning to specific user
user2.addWarning(); // Add one warning to specific user
user2.getUserData(); // Returning data user object

Reference: Practical usage of closures
In practice, closures may create elegant designs, allowing customization of various calculations, deferred calls, callbacks, creating encapsulated scope, etc.
An example is the sort method of arrays which accepts the sort condition function as an argument:
[1, 2, 3].sort(function (a, b) {
... // Sort conditions
});
Mapping functionals as the map method of arrays which maps a new array by the condition of the functional argument:
[1, 2, 3].map(function (element) {
return element * 2;
}); // [2, 4, 6]
Often it is convenient to implement search functions with using functional arguments defining almost unlimited conditions for search:
someCollection.find(function (element) {
return element.someProperty == 'searchCondition';
});
Also, we may note applying functionals as, for example, a forEach method which applies a function to an array of elements:
[1, 2, 3].forEach(function (element) {
if (element % 2 != 0) {
alert(element);
}
}); // 1, 3
A function is applied to arguments (to a list of arguments — in apply, and to positioned arguments — in call):
(function () {
alert([].join.call(arguments, ';')); // 1;2;3
}).apply(this, [1, 2, 3]);
Deferred calls:
var a = 10;
setTimeout(function () {
alert(a); // 10, after one second
}, 1000);
Callback functions:
var x = 10;
// Only for example
xmlHttpRequestObject.onreadystatechange = function () {
// Callback, which will be called deferral ,
// when data will be ready;
// variable "x" here is available,
// regardless that context in which,
// it was created already finished
alert(x); // 10
};
Creation of an encapsulated scope for the purpose of hiding auxiliary objects:
var foo = {};
(function (object) {
var x = 10;
object.getX = function _getX() {
return x;
};
})(foo);
alert(foo.getX()); // Get closured "x" – 10

In the given sample, the value of the enclosed variable 'counter' is protected and can be altered only using the given functions (increment, decrement). Because it is in a closure,
var MyCounter = function (){
var counter = 0;
return {
increment:function () {return counter += 1;},
decrement:function () {return counter -= 1;},
get:function () {return counter;}
};
};
var x = MyCounter();
// Or
var y = MyCounter();
alert(x.get()); // 0
alert(x.increment()); // 1
alert(x.increment()); // 2
alert(y.increment()); // 1
alert(x.get()); // x is still 2

Everyone has explained the practical use cases of closure: the definition and a couple of examples.
I want to contribute a list of use cases of Closures:
suppose you want to count no of times a button is clicked; Closure is the best choice.
Throttling and Debounce
Currying
Memorize
Maintaining state in the async world
Functions like once
setTimeouts
Iterators

what are some common uses of closures? [duplicate]

I'm trying my hardest to wrap my head around JavaScript closures.
I get that by returning an inner function, it will have access to any variable defined in its immediate parent.
Where would this be useful to me? Perhaps I haven't quite got my head around it yet. Most of the examples I have seen online don't provide any real world code, just vague examples.
Can someone show me a real world use of a closure?
Is this one, for example?
var warnUser = function (msg) {
var calledCount = 0;
return function() {
calledCount++;
alert(msg + '\nYou have been warned ' + calledCount + ' times.');
};
};
var warnForTamper = warnUser('You can not tamper with our HTML.');
warnForTamper();
warnForTamper();

Suppose, you want to count the number of times user clicked a button on a webpage.
For this, you are triggering a function on onclick event of button to update the count of the variable
<button onclick="updateClickCount()">click me</button>
Now there could be many approaches like:
You could use a global variable, and a function to increase the counter:
var counter = 0;
function updateClickCount() {
++counter;
// Do something with counter
}
But, the pitfall is that any script on the page can change the counter, without calling updateClickCount().
Now, you might be thinking of declaring the variable inside the function:
function updateClickCount() {
var counter = 0;
++counter;
// Do something with counter
}
But, hey! Every time updateClickCount() function is called, the counter is set to 1 again.
Thinking about nested functions?
Nested functions have access to the scope "above" them.
In this example, the inner function updateClickCount() has access to the counter variable in the parent function countWrapper():
function countWrapper() {
var counter = 0;
function updateClickCount() {
++counter;
// Do something with counter
}
updateClickCount();
return counter;
}
This could have solved the counter dilemma, if you could reach the updateClickCount() function from the outside and you also need to find a way to execute counter = 0 only once not everytime.
Closure to the rescue! (self-invoking function):
var updateClickCount = (function(){
var counter = 0;
return function(){
++counter;
// Do something with counter
}
})();
The self-invoking function only runs once. It sets the counter to zero (0), and returns a function expression.
This way updateClickCount becomes a function. The "wonderful" part is that it can access the counter in the parent scope.
This is called a JavaScript closure. It makes it possible for a function to have "private" variables.
The counter is protected by the scope of the anonymous function, and can only be changed using the updateClickCount() function!
A more lively example on closures
<script>
var updateClickCount = (function(){
var counter = 0;
return function(){
++counter;
document.getElementById("spnCount").innerHTML = counter;
}
})();
</script>
<html>
<button onclick="updateClickCount()">click me</button>
<div> you've clicked
<span id="spnCount"> 0 </span> times!
</div>
</html>
Reference: JavaScript Closures

I've used closures to do things like:
a = (function () {
var privatefunction = function () {
alert('hello');
}
return {
publicfunction : function () {
privatefunction();
}
}
})();
As you can see there, a is now an object, with a method publicfunction ( a.publicfunction() ) which calls privatefunction, which only exists inside the closure. You can not call privatefunction directly (i.e. a.privatefunction() ), just publicfunction().
It's a minimal example, but maybe you can see uses to it? We used this to enforce public/private methods.

The example you give is an excellent one. Closures are an abstraction mechanism that allow you to separate concerns very cleanly. Your example is a case of separating instrumentation (counting calls) from semantics (an error-reporting API). Other uses include:
Passing parameterised behaviour into an algorithm (classic higher-order programming):
function proximity_sort(arr, midpoint) {
arr.sort(function(a, b) { a -= midpoint; b -= midpoint; return a*a - b*b; });
}
Simulating object oriented programming:
function counter() {
var a = 0;
return {
inc: function() { ++a; },
dec: function() { --a; },
get: function() { return a; },
reset: function() { a = 0; }
}
}
Implementing exotic flow control, such as jQuery's Event handling and AJAX APIs.

JavaScript closures can be used to implement throttle and debounce functionality in your application.
Throttling
Throttling puts a limit on as a maximum number of times a function can be called over time. As in "execute this function at most once every 100 milliseconds."
Code:
const throttle = (func, limit) => {
let isThrottling
return function() {
const args = arguments
const context = this
if (!isThrottling) {
func.apply(context, args)
isThrottling = true
setTimeout(() => isThrottling = false, limit)
}
}
}
Debouncing
Debouncing puts a limit on a function not be called again until a certain amount of time has passed without it being called. As in "execute this function only if 100 milliseconds have passed without it being called."
Code:
const debounce = (func, delay) => {
let debouncing
return function() {
const context = this
const args = arguments
clearTimeout(debouncing)
debouncing = setTimeout(() => func.apply(context, args), delay)
}
}
As you can see closures helped in implementing two beautiful features which every web application should have to provide smooth UI experience functionality.

Yes, that is a good example of a useful closure. The call to warnUser creates the calledCount variable in its scope and returns an anonymous function which is stored in the warnForTamper variable. Because there is still a closure making use of the calledCount variable, it isn't deleted upon the function's exit, so each call to the warnForTamper() will increase the scoped variable and alert the value.
The most common issue I see on Stack Overflow is where someone wants to "delay" use of a variable that is increased upon each loop, but because the variable is scoped then each reference to the variable would be after the loop has ended, resulting in the end state of the variable:
for (var i = 0; i < someVar.length; i++)
window.setTimeout(function () {
alert("Value of i was "+i+" when this timer was set" )
}, 10000);
This would result in every alert showing the same value of i, the value it was increased to when the loop ended. The solution is to create a new closure, a separate scope for the variable. This can be done using an instantly executed anonymous function, which receives the variable and stores its state as an argument:
for (var i = 0; i < someVar.length; i++)
(function (i) {
window.setTimeout(function () {
alert("Value of i was " + i + " when this timer was set")
}, 10000);
})(i);

In the JavaScript (or any ECMAScript) language, in particular, closures are useful in hiding the implementation of functionality while still revealing the interface.
For example, imagine you are writing a class of date utility methods and you want to allow users to look up weekday names by index, but you don't want them to be able to modify the array of names you use under the hood.
var dateUtil = {
weekdayShort: (function() {
var days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'];
return function(x) {
if ((x != parseInt(x)) || (x < 1) || (x > 7)) {
throw new Error("invalid weekday number");
}
return days[x - 1];
};
}())
};
Note that the days array could simply be stored as a property of the dateUtil object, but then it would be visible to users of the script and they could even change it if they wanted, without even needing your source code. However, since it's enclosed by the anonymous function which returns the date lookup function it is only accessible by the lookup function so it is now tamperproof.

There is a section on Practical Closures at the Mozilla Developer Network.

If you're comfortable with the concept of instantiating a class in the object-oriented sense (i.e. to create an object of that class) then you're close to understanding closures.
Think of it this way: when you instantiate two Person objects you know that the class member variable "Name" is not shared between instances; each object has its own 'copy'. Similarly, when you create a closure, the free variable ('calledCount' in your example above) is bound to the 'instance' of the function.
I think your conceptual leap is slightly hampered by the fact that every function/closure returned by the warnUser function (aside: that's a higher-order function) closure binds 'calledCount' with the same initial value (0), whereas often when creating closures it is more useful to pass different initializers into the higher-order function, much like passing different values to the constructor of a class.
So, suppose when 'calledCount' reaches a certain value you want to end the user's session; you might want different values for that depending on whether the request comes in from the local network or the big bad internet (yes, it's a contrived example). To achieve this, you could pass different initial values for calledCount into warnUser (i.e. -3, or 0?).
Part of the problem with the literature is the nomenclature used to describe them ("lexical scope", "free variables"). Don't let it fool you, closures are more simple than would appear... prima facie ;-)

Here, I have a greeting that I want to say several times. If I create a closure, I can simply call that function to record the greeting. If I don't create the closure, I have to pass my name in every single time.
Without a closure (https://jsfiddle.net/lukeschlangen/pw61qrow/3/):
function greeting(firstName, lastName) {
var message = "Hello " + firstName + " " + lastName + "!";
console.log(message);
}
greeting("Billy", "Bob");
greeting("Billy", "Bob");
greeting("Billy", "Bob");
greeting("Luke", "Schlangen");
greeting("Luke", "Schlangen");
greeting("Luke", "Schlangen");
With a closure (https://jsfiddle.net/lukeschlangen/Lb5cfve9/3/):
function greeting(firstName, lastName) {
var message = "Hello " + firstName + " " + lastName + "!";
return function() {
console.log(message);
}
}
var greetingBilly = greeting("Billy", "Bob");
var greetingLuke = greeting("Luke", "Schlangen");
greetingBilly();
greetingBilly();
greetingBilly();
greetingLuke();
greetingLuke();
greetingLuke();

Another common use for closures is to bind this in a method to a specific object, allowing it to be called elsewhere (such as as an event handler).
function bind(obj, method) {
if (typeof method == 'string') {
method = obj[method];
}
return function () {
method.apply(obj, arguments);
}
}
...
document.body.addEventListener('mousemove', bind(watcher, 'follow'), true);
Whenever a mousemove event fires, watcher.follow(evt) is called.
Closures are also an essential part of higher-order functions, allowing the very common pattern of rewriting multiple similar functions as a single higher order function by parameterizing the dissimilar portions. As an abstract example,
foo_a = function (...) {A a B}
foo_b = function (...) {A b B}
foo_c = function (...) {A c B}
becomes
fooer = function (x) {
return function (...) {A x B}
}
where A and B aren't syntactical units but source code strings (not string literals).
See "Streamlining my javascript with a function" for a concrete example.

Here I have one simple example of the closure concept which we can use for in our E-commerce site or many others as well.
I am adding my JSFiddle link with the example. It contains a small product list of three items and one cart counter.
JSFiddle
// Counter closure implemented function;
var CartCouter = function(){
var counter = 0;
function changeCounter(val){
counter += val
}
return {
increment: function(){
changeCounter(1);
},
decrement: function(){
changeCounter(-1);
},
value: function(){
return counter;
}
}
}
var cartCount = CartCouter();
function updateCart() {
document.getElementById('cartcount').innerHTML = cartCount.value();
}
var productlist = document.getElementsByClassName('item');
for(var i = 0; i< productlist.length; i++){
productlist[i].addEventListener('click', function(){
if(this.className.indexOf('selected') < 0){
this.className += " selected";
cartCount.increment();
updateCart();
}
else{
this.className = this.className.replace("selected", "");
cartCount.decrement();
updateCart();
}
})
}
.productslist{
padding: 10px;
}
ul li{
display: inline-block;
padding: 5px;
border: 1px solid #DDD;
text-align: center;
width: 25%;
cursor: pointer;
}
.selected{
background-color: #7CFEF0;
color: #333;
}
.cartdiv{
position: relative;
float: right;
padding: 5px;
box-sizing: border-box;
border: 1px solid #F1F1F1;
}
<div>
<h3>
Practical use of a JavaScript closure concept/private variable.
</h3>
<div class="cartdiv">
<span id="cartcount">0</span>
</div>
<div class="productslist">
<ul>
<li class="item">Product 1</li>
<li class="item">Product 2</li>
<li class="item">Product 3</li>
</ul>
</div>
</div>

Use of Closures:
Closures are one of the most powerful features of JavaScript. JavaScript allows for the nesting of functions and grants the inner function full access to all the variables and functions defined inside the outer function (and all other variables and functions that the outer function has access to). However, the outer function does not have access to the variables and functions defined inside the inner function.
This provides a sort of security for the variables of the inner function. Also, since the inner function has access to the scope of the outer function, the variables and functions defined in the outer function will live longer than the outer function itself, if the inner function manages to survive beyond the life of the outer function. A closure is created when the inner function is somehow made available to any scope outside the outer function.
Example:
<script>
var createPet = function(name) {
var sex;
return {
setName: function(newName) {
name = newName;
},
getName: function() {
return name;
},
getSex: function() {
return sex;
},
setSex: function(newSex) {
if(typeof newSex == "string" && (newSex.toLowerCase() == "male" || newSex.toLowerCase() == "female")) {
sex = newSex;
}
}
}
}
var pet = createPet("Vivie");
console.log(pet.getName()); // Vivie
console.log(pet.setName("Oliver"));
console.log(pet.setSex("male"));
console.log(pet.getSex()); // male
console.log(pet.getName()); // Oliver
</script>
In the code above, the name variable of the outer function is accessible to the inner functions, and there is no other way to access the inner variables except through the inner functions. The inner variables of the inner function act as safe stores for the inner functions. They hold "persistent", yet secure, data for the inner functions to work with. The functions do not even have to be assigned to a variable, or have a name.
read here for detail.

Explaining the practical use for a closure in JavaScript
When we create a function inside another function, we are creating a closure. Closures are powerful because they are capable of reading and manipulating the data of its outer functions. Whenever a function is invoked, a new scope is created for that call. The local variable declared inside the function belong to that scope and they can only be accessed from that function. When the function has finished the execution, the scope is usually destroyed.
A simple example of such function is this:
function buildName(name) {
const greeting = "Hello, " + name;
return greeting;
}
In above example, the function buildName() declares a local variable greeting and returns it. Every function call creates a new scope with a new local variable. After the function is done executing, we have no way to refer to that scope again, so it’s garbage collected.
But how about when we have a link to that scope?
Let’s look at the next function:
function buildName(name) {
const greeting = "Hello, " + name + " Welcome ";
const sayName = function() {
console.log(greeting);
};
return sayName;
}
const sayMyName = buildName("Mandeep");
sayMyName(); // Hello, Mandeep Welcome
The function sayName() from this example is a closure. The sayName() function has its own local scope (with variable welcome) and has also access to the outer (enclosing) function’s scope. In this case, the variable greeting from buildName().
After the execution of buildName is done, the scope is not destroyed in this case. The sayMyName() function still has access to it, so it won’t be garbage collected. However, there is no other way of accessing data from the outer scope except the closure. The closure serves as the gateway between the global context and the outer scope.

The JavaScript module pattern uses closures. Its nice pattern allows you to have something alike "public" and "private" variables.
var myNamespace = (function () {
var myPrivateVar, myPrivateMethod;
// A private counter variable
myPrivateVar = 0;
// A private function which logs any arguments
myPrivateMethod = function(foo) {
console.log(foo);
};
return {
// A public variable
myPublicVar: "foo",
// A public function utilizing privates
myPublicFunction: function(bar) {
// Increment our private counter
myPrivateVar++;
// Call our private method using bar
myPrivateMethod(bar);
}
};
})();

I like Mozilla's function factory example.
function makeAdder(x) {
return function(y) {
return x + y;
};
}
var addFive = makeAdder(5);
console.assert(addFive(2) === 7);
console.assert(addFive(-5) === 0);

This thread has helped me immensely in gaining a better understanding of how closures work.
I've since done some experimentation of my own and came up with this fairly simple code which may help some other people see how closures can be used in a practical way and how to use the closure at different levels to maintain variables similar to static and/or global variables without risk of them getting overwritten or confused with global variables.
This keeps track of button clicks, both at a local level for each individual button and a global level, counting every button click, contributing towards a single figure. Note I haven't used any global variables to do this, which is kind of the point of the exercise - having a handler that can be applied to any button that also contributes to something globally.
Please experts, do let me know if I've committed any bad practices here! I'm still learning this stuff myself.
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Closures on button presses</title>
<script type="text/javascript">
window.addEventListener("load" , function () {
/*
Grab the function from the first closure,
and assign to a temporary variable
this will set the totalButtonCount variable
that is used to count the total of all button clicks
*/
var buttonHandler = buttonsCount();
/*
Using the result from the first closure (a function is returned)
assign and run the sub closure that carries the
individual variable for button count and assign to the click handlers
*/
document.getElementById("button1").addEventListener("click" , buttonHandler() );
document.getElementById("button2").addEventListener("click" , buttonHandler() );
document.getElementById("button3").addEventListener("click" , buttonHandler() );
// Now that buttonHandler has served its purpose it can be deleted if needs be
buttonHandler = null;
});
function buttonsCount() {
/*
First closure level
- totalButtonCount acts as a sort of global counter to count any button presses
*/
var totalButtonCount = 0;
return function () {
// Second closure level
var myButtonCount = 0;
return function (event) {
// Actual function that is called on the button click
event.preventDefault();
/*
Increment the button counts.
myButtonCount only exists in the scope that is
applied to each event handler and therefore acts
to count each button individually, whereas because
of the first closure totalButtonCount exists at
the scope just outside, it maintains a sort
of static or global variable state
*/
totalButtonCount++;
myButtonCount++;
/*
Do something with the values ... fairly pointless
but it shows that each button contributes to both
its own variable and the outer variable in the
first closure
*/
console.log("Total button clicks: "+totalButtonCount);
console.log("This button count: "+myButtonCount);
}
}
}
</script>
</head>
<body>
Button 1
Button 2
Button 3
</body>
</html>

There are various use cases of closures.Here, I am going to explain most important usage of Closure concept.
Closure can be used to create private methods and variables just like an object-oriented language like java, c++ and so on. Once you implemented private methods and variables, your variables defined inside a function won't be accessible by window object. This helps in data hiding and data security.
const privateClass = () => {
let name = "sundar";
function setName(changeName) {
name = changeName;
}
function getName() {
return name;
}
return {
setName: setName,
getName: getName,
};
};
let javaLikeObject = privateClass(); \\ similar to new Class() in OOPS.
console.log(javaLikeObject.getName()); \\this will give sundar
javaLikeObject.setName("suresh");
console.log(javaLikeObject.getName()); \\this will give suresh
Another real-life example of closure :
Create index.html:
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Program with Javascript</title>
</head>
<body>
<p id="first"></p>
<p id="second"></p>
<button onclick="applyingConcepts()">Click</button>
<script src="./index.js"></script>
</body>
</html>
2)In index.js:
let count = 0;
return () => {
document.getElementById("first").innerHTML = count++;
};
})();
In this example, when you click a button, then your count will be updated on p#id.
Note: You might be wondering what's special in this code. When you inspect, you will notice that you can't change the value of count using the window object. This means you have declared private variable count so this prevents your states from being spoiled by the client.

I wrote an article a while back about how closures can be used to simplify event-handling code. It compares ASP.NET event handling to client-side jQuery.
http://www.hackification.com/2009/02/20/closures-simplify-event-handling-code/

Much of the code we write in front-end JavaScript is event-based — we define some behavior, then attach it to an event that is triggered by the user (such as a click or a keypress). Our code is generally attached as a callback: a single function which is executed in response to the event.
size12, size14, and size16 are now functions which will resize the body text to 12, 14, and 16 pixels, respectively. We can attach them to buttons (in this case links) as follows:
function makeSizer(size) {
return function() {
document.body.style.fontSize = size + 'px';
};
}
var size12 = makeSizer(12);
var size14 = makeSizer(14);
var size16 = makeSizer(16);
document.getElementById('size-12').onclick = size12;
document.getElementById('size-14').onclick = size14;
document.getElementById('size-16').onclick = size16;
Fiddle

Closures are a useful way to create generators, a sequence incremented on-demand:
var foobar = function(i){var count = count || i; return function(){return ++count;}}
baz = foobar(1);
console.log("first call: " + baz()); //2
console.log("second call: " + baz()); //3
The differences are summarized as follows:
Anonymous functions Defined functions
Cannot be used as a method Can be used as a method of an object
Exists only in the scope in which it is defined Exists within the object it is defined in
Can only be called in the scope in which it is defined Can be called at any point in the code
Can be reassigned a new value or deleted Cannot be deleted or changed
References
AS3 Fundamentals: Functions

I'm trying to learn closures and I think the example that I have created is a practical use case. You can run a snippet and see the result in the console.
We have two separate users who have separate data. Each of them can see the actual state and update it.
function createUserWarningData(user) {
const data = {
name: user,
numberOfWarnings: 0,
};
function addWarning() {
data.numberOfWarnings = data.numberOfWarnings + 1;
}
function getUserData() {
console.log(data);
return data;
}
return {
getUserData: getUserData,
addWarning: addWarning,
};
}
const user1 = createUserWarningData("Thomas");
const user2 = createUserWarningData("Alex");
//USER 1
user1.getUserData(); // Returning data user object
user1.addWarning(); // Add one warning to specific user
user1.getUserData(); // Returning data user object
//USER2
user2.getUserData(); // Returning data user object
user2.addWarning(); // Add one warning to specific user
user2.addWarning(); // Add one warning to specific user
user2.getUserData(); // Returning data user object

Reference: Practical usage of closures
In practice, closures may create elegant designs, allowing customization of various calculations, deferred calls, callbacks, creating encapsulated scope, etc.
An example is the sort method of arrays which accepts the sort condition function as an argument:
[1, 2, 3].sort(function (a, b) {
... // Sort conditions
});
Mapping functionals as the map method of arrays which maps a new array by the condition of the functional argument:
[1, 2, 3].map(function (element) {
return element * 2;
}); // [2, 4, 6]
Often it is convenient to implement search functions with using functional arguments defining almost unlimited conditions for search:
someCollection.find(function (element) {
return element.someProperty == 'searchCondition';
});
Also, we may note applying functionals as, for example, a forEach method which applies a function to an array of elements:
[1, 2, 3].forEach(function (element) {
if (element % 2 != 0) {
alert(element);
}
}); // 1, 3
A function is applied to arguments (to a list of arguments — in apply, and to positioned arguments — in call):
(function () {
alert([].join.call(arguments, ';')); // 1;2;3
}).apply(this, [1, 2, 3]);
Deferred calls:
var a = 10;
setTimeout(function () {
alert(a); // 10, after one second
}, 1000);
Callback functions:
var x = 10;
// Only for example
xmlHttpRequestObject.onreadystatechange = function () {
// Callback, which will be called deferral ,
// when data will be ready;
// variable "x" here is available,
// regardless that context in which,
// it was created already finished
alert(x); // 10
};
Creation of an encapsulated scope for the purpose of hiding auxiliary objects:
var foo = {};
(function (object) {
var x = 10;
object.getX = function _getX() {
return x;
};
})(foo);
alert(foo.getX()); // Get closured "x" – 10

In the given sample, the value of the enclosed variable 'counter' is protected and can be altered only using the given functions (increment, decrement). Because it is in a closure,
var MyCounter = function (){
var counter = 0;
return {
increment:function () {return counter += 1;},
decrement:function () {return counter -= 1;},
get:function () {return counter;}
};
};
var x = MyCounter();
// Or
var y = MyCounter();
alert(x.get()); // 0
alert(x.increment()); // 1
alert(x.increment()); // 2
alert(y.increment()); // 1
alert(x.get()); // x is still 2

Everyone has explained the practical use cases of closure: the definition and a couple of examples.
I want to contribute a list of use cases of Closures:
suppose you want to count no of times a button is clicked; Closure is the best choice.
Throttling and Debounce
Currying
Memorize
Maintaining state in the async world
Functions like once
setTimeouts
Iterators

Why are objects' values captured inside function calls?

This code is supposed to pop up an alert with the number of the image when you click it:
for(var i=0; i<10; i++) {
$("#img" + i).click(
function () { alert(i); }
);
}
You can see it not working at http://jsfiddle.net/upFaJ/. I know that this is because all of the click-handler closures are referring to the same object i, so every single handler pops up "10" when it's triggered.
However, when I do this, it works fine:
for(var i=0; i<10; i++) {
(function (i2) {
$("#img" + i2).click(
function () { alert(i2); }
);
})(i);
}
You can see it working at http://jsfiddle.net/v4sSD/.
Why does it work? There's still only one i object in memory, right? Objects are always passed by reference, not copied, so the self-executing function call should make no difference. The output of the two code snippets should be identical. So why is the i object being copied 10 times? Why does it work?
I think it's interesting that this version doesn't work:
for(var i=0; i<10; i++) {
(function () {
$("#img" + i).click(
function () { alert(i); }
);
})();
}
It seems that the passing of the object as a function parameter makes all the difference.
EDIT: OK, so the previous example can be explained by primitives (i) being passed by value to the function call. But what about this example, which uses real objects?
for(var i=0; i<5; i++) {
var toggler = $("<img/>", { "src": "http://www.famfamfam.com/lab/icons/silk/icons/cross.png" });
toggler.click(function () { toggler.attr("src", "http://www.famfamfam.com/lab/icons/silk/icons/tick.png"); });
$("#container").append(toggler);
}
Not working: http://jsfiddle.net/Zpwku/
for(var i=0; i<5; i++) {
var toggler = $("<img/>", { "src": "http://www.famfamfam.com/lab/icons/silk/icons/cross.png" });
(function (t) {
t.click(function () { t.attr("src", "http://www.famfamfam.com/lab/icons/silk/icons/tick.png"); });
$("#container").append(t);
})(toggler);
}
Working: http://jsfiddle.net/YLSn6/

Most of the answers are correct in that passing an object as a function parameter breaks a closure and thus allow us to assign things to functions from within a loop. But I'd like to point out why this is the case, and it's not just a special case for closures.
You see, the way javascript passes parameters to functions is a bit different form other languages. Firstly, it seems to have two ways of doing it depending on weather it's a primitive value or an object. For primitive values it seems to pass by value and for objects it seems to pass by reference.
How javascript passes function arguments
Actually, the real explanation of what javascript does explains both situations, as well as why it breaks closures, using just a single mechanism.
What javascript does is actually it passes parameters by copy of reference. That is to say, it creates another reference to the parameter and passes that new reference into the function.
Pass by value?
Assume that all variables in javascript are references. In other languages, when we say a variable is a reference, we expect it to behave like this:
var i = 1;
function increment (n) { n = n+1 };
increment(i); // we would expect i to be 2 if i is a reference
But in javascript, it's not the case:
console.log(i); // i is still 1
That's a classic pass by value isn't it?
Pass by reference?
But wait, for objects it's a different story:
var o = {a:1,b:2}
function foo (x) {
x.c = 3;
}
foo(o);
If parameters were passed by value we'd expect the o object to be unchanged but:
console.log(o); // outputs {a:1,b:2,c:3}
That's classic pass by reference there. So we have two behaviors depending on weather we're passing a primitive type or an object.
Wait, what?
But wait a second, check this out:
var o = {a:1,b:2,c:3}
function bar (x) {
x = {a:2,b:4,c:6}
}
bar(o);
Now see what happens:
console.log(o); // outputs {a:1,b:2,c:3}
What! That's not passing by reference! The values are unchanged!
Which is why I call it pass by copy of reference. If we think about it this way, everything makes sense. We don't need to think of primitives as having special behavior when passed into a function because objects behave the same way. If we try to modify the object the variable points to then it works like pass by reference but if we try to modify the reference itself then it works like pass by value.
This also explains why closures are broken by passing a variable as a function parameter. Because the function call will create another reference that is not bound by the closure like the original variable.
Epilogue: I lied
One more thing before we end this. I said before that this unifies the behavior of primitive types and objects. Actually no, primitive types are still different:
var i = 1;
function bat (n) { n.hello = 'world' };
bat(i);
console.log(i.hello); // undefined, i is unchanged
I give up. There's no making sense of this. It's just the way it is.

It's because you are calling a function, passing it a value.
for (var i = 0; i < 10; i++) {
alert(i);
}
You expect this to alert different values, right? Because you are passing the current value of i to alert.
function attachClick(val) {
$("#img" + val).click(
function () { alert(val); }
);
}
With this function, you'd expect it to alert whatever val was passed into it, right? That also works when calling it in a loop:
for (var i = 0; i < 10; i++) {
attachClick(i);
}
This:
for (var i = 0; i < 10; i++) {
(function (val) {
$("#img" + val).click(
function () { alert(val); }
);
})(i);
}
is just an inline declaration of the above. You are declaring an anonymous function with the same characteristics as attachClick above and you call it immediately. The act of passing a value through a function parameter breaks any references to the i variable.

upvoted deceze's answer, but thought I'd try a simpler explanation. The reason the closure works is that variables in javascript are function scoped. The closure creates a new scope, and by passing the value of i in as a parameter, you are defining a local variable i in the new scope. without the closure, all of the click handlers you define are in the same scope, using the same i. the reason that your last code snippet doesn't work is because there is no local i, so all click handlers are looking to the nearest parent context with i defined.
I think the other thing that might be confusing you is this comment
Objects are always passed by reference, not copied, so the self-executing function call should make no difference.
this is true for objects, but not primitive values (numbers, for example). This is why a new local i can be defined. To demonstrate, if you did something weird like wrapping the value of i in an array, the closure would not work, because arrays are passed by reference.
// doesn't work
for(var i=[0]; i[0]<10; i[0]++) {
(function (i2) {
$("#img" + i2[0]).click(
function () { alert(i2[0]); }
);
})(i);
}

In the first example, there is only one value of i and it's the one used in the for loop. This, all event handlers will show the value of i when the for loop ends, not the desired value.
In the second example, the value of i at the time the event handler is installed is copied to the i2 function argument and there is a separate copy of that for each invocation of the function and thus for each event handler.
So, this:
(function (i2) {
$("#img" + i2).click(
function () { alert(i2); }
);
})(i);
Creates a new variable i2 that has it's own value for each separate invocation of the function. Because of closures in javascript, each separate copy of i2 is preserved for each separate event handler - thus solving your problem.
In the third example, no new copy of i is made (they all refer to the same i from the for loop) so it works the same as the first example.

Code 1 and Code 3 didn't work because i is a variable and values are changed in each loop. At the end of loop 10 will be assigned to i.
For more clear, take a look at this example,
for(var i=0; i<10; i++) {
}
alert(i)
http://jsfiddle.net/muthkum/t4Ur5/
You can see I put a alert after the loop and it will show show alert box with value 10.
This is what happening to Code 1 and Code 3.

Run the next example:
for(var i=0; i<10; i++) {
$("#img" + i).click(
function () { alert(i); }
);
}
i++;
You'll see that now, 11 is being alerted.
Therefore, you need to avoid the reference to i, by sending it as a function parameter, by it's value. You have already found the solution.

One thing that the other answers didn't mention is why this example that I gave in the question doesn't work:
for(var i=0; i<5; i++) {
var toggler = $("<img/>", { "src": "http://www.famfamfam.com/lab/icons/silk/icons/cross.png" });
toggler.click(function () { toggler.attr("src", "http://www.famfamfam.com/lab/icons/silk/icons/tick.png"); });
$("#container").append(toggler);
}
Coming back to the question months later with a better understanding of JavaScript, the reason it doesn't work can be understood as follows:
The var toggler declaration is hoisted to the top of the function call. All references to toggler are to the same actual identifier.
The closure referenced in the anonymous function is the same (not a shallow copy) of the one containing toggler, which is being updated for each iteration of the loop.
#2 is quite surprising. This alerts "5" for example:
var o;
setTimeout(function () { o = {value: 5}; }, 100);
setTimeout(function () { alert(o.value) }, 1000);

JavaScript: is there any better way to do a for loop with callback function?

for(var i=1; i<496; i++) {
(function(num) {
myApp.getTerm(num, function (term, def){
myApp.quizlet[0].terms[num] = { term: term, definition: def};
});
})(i);
};
I'm calling a function with callback inside a for-loop. The callback function need to access the current loop's iteration i. I'm coming up with the solution above.
Is there any other way to do things like this?

It's more readable to define a named function to call:
myApp.getTermForQuizzlet = function(num){
myApp.getTerm(num, function (term, def){
myApp.quizlet[0].terms[num] = { term: term, definition: def };
});
};
for(var i=1; i<496; i++) {
myApp.getTermForQuizzlet(i);
};
Other than that, I don't know what you can do.

What you're doing is not unreasonable and should work fine. Purely at the level of readability, I might go with
for(var i=1; i<496; i++) {
function get_callback(n) {
return function(term, def) {
myApp.quizlet[0].terms[n] = { term: term, definition: def};
};
}
myApp.getTerm(i, get_callback(i));
};
If you are comfortable using Function.bind:
function callback (n, term, def) {
myApp.quizlet[0].terms[n] = { term: term, definition: def};
}
for(var i=1; i<496; i++) {
myApp.getTerm(i, callback.bind(this,i));
}
We bind the first argument to i, resulting in a "curried" function taking the term and def arguments.
Then of course there is this horrible hack, do not try this at home.
for(var i=1; i<496; i++) {
try { throw i; }
catch (i) {
myApp.getTerm(i, function (term,def) {
myApp.quizlet[0].terms[i] = { term: term, definition: def};
});
}
}

Most of the time I use iteration functions instead of bulidtin loops. They are available in newer browsers and most JS libraries:
array.forEach(function(item, i){
//We can freely use "item" and "i" in callbacks without worrying
setTimeout( function(){ console.log(item, i) }, 1000 );
});
This it is usually shorter the the for-loop version (if you are iterating over an array at least) and it avoids the closures-in-for-loops bug for "free", without having to add extra imediately invoked functions or things like that.
The major downside of this method is that it doesn't support using the "this" and using break, continue and return statements as well as the builtin loops and that most libraries come with only functions for array or dictionary iteration, without support for more general loop styles. While I rarelly end up needing this kind of advanced functionality, when it does come up I usually just use extra closures (like you did) or, if it is a common pattern, write my own iterator functions.
var forLoop = function(i0, n, f){
for(var i=i0; i<n; i++){
f(i);
}
};
forLoop(1, 496, function(i){
myApp.getTerm(i, function(term, def){
myApp.quizlet[0].terms[i] = { ... }
});
});

The structure inside the loop is called Immediately-Invoked Function Expression (IIFE).
IIFEs can be useful for example when you create event handlers in a cycle, and would like to lock the current value of the loop variable for each event handler for later use. This "locking" is called (lexical or function) closure: the lexical scope of the outer function (in your case, the IIFE), including any constants, local variables and argument values (in your case, the num argument of the IIFE, which is the actual value of the loop variable), become part of the internal state of each inner function object (in your case, the callback function passed to myApp.getTerm), even after execution of the outer function (the IIFE) concludes.
So I think you do this right, using an IIFE is a perfectly valid solution in your case.
An intersting example of this pattern (IIFE for closure) can be found in John Resig's implementation of inheritance:
// ...
(function(name, fn){
return function() {
// use name and fn here
};
})(name, prop[name])
// ...

It seems as though you basically wish to
Create app, myApp;
Add property .quiz to myApp;
Add property .terms to .quiz;
For each of the terms in the quiz, form an object to relate each term to it's definition.
To set up the properties you need you might do something like this ..
var terms = ['js', 'php', 'css', 'html', 'as3'];
var defin = ['about js', 'about php', 'about css', 'about html', 'about as3'];
var myApp = {};
myApp.quiz = new Array;
myApp.quiz[0] = [];
myApp.quiz[0].terms = new Array;
terms.forEach(function(ele, idx){
myApp.quiz[0].terms[idx] = {term: terms[idx], def: defin[idx]};
})
Now you need a way to access any term in any quiz. So you could do that with a function such as ..
function getTerm(qn, tn){
return myApp.quiz[qn].terms[tn];
}
var q0t2 = getTerm(0, 2);
console.log(q0t2); // def: "about css", term: "css"
I am not so sure just what your needs are here, but this is my best guess.
Please elaborate if your situation does not fit the sort of methods I am using here. :)

Defining and calling function in one step

Is there a way in Javascript to define a function and immediately call it, in a way that allows it to be reused?
I know you can do one-off anonymous functions:
(function(i) {
var product = i * i;
console.log(product);
// Can't recurse here because there's no (ECMA standard) way for the
// function to refer to itself
}(2)); // logs 4
Or you can name a function then call it afterwards:
function powers(i) {
var product = i * i;
console.log(i * i);
if (product < 1e6) { powers(product) };
}
powers(2); // Logs 4, 16, 256...
But is there a cleaner way of defining and calling a function in one go? Sort of like a hybrid of both examples?
Not being able to do this isn't preventing me from doing anything, but it feels like it would be a nice expressive way to write recursive functions or functions that need to be run on $(document).ready() but also later when situations change, etc.

You can try:
(window.powers = function(i) {
/*Code here*/
alert('test : ' + i);
})(2);
Click
Working link : http://jsfiddle.net/SqBp8/
It gets called on load, and I have added it to an anchor tag to change the parameter and alert.

If all you want is access the function within its own body, you can simply specify a name after the function keyword:
> (function fac (n) {
return (n === 0 ? 1 : n*fac(n-1));
})(10)
3628800
This is a standard feature (see ECMA-262, ed. 5.1, p. 98).

All the answers here are close to what you want, but have a few problems (adding it to the global scope, not actually calling it, etc). This combines a few examples on this page (although it unfortunately requires you to remember arguments.callee):
var test = (function() {
alert('hi');
return arguments.callee;
})();
Later, you can call it:
test();

If you don't care about the return value, you can do this.
var powers = function powers(i) {
var product = i * i;
console.log(i * i);
if (product < 1e6) { powers(product) };
return powers;
}(2);

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