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javascript closure for_loop [duplicate]

This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 6 years ago.
I have read a number of explanations about closures and closures inside loops. I have a hard time understanding the concept. I have this code: Is there a way to reduce the code as much as possible so the concept of closure can be made clearer. I am having a hard time understanding the part in which the i is inside two parenthesis. Thanks
function addLinks () {
for (var i=0, link; i<5; i++) {
link = document.createElement("a");
link.innerHTML = "Link " + i;
link.onclick = function (num) {
return function () {
alert(num);
};
}(i);
document.body.appendChild(link);
}
}
window.onload = addLinks;

WARNING: Long(ish) Answer
This is copied directly from an article I wrote in an internal company wiki:
Question: How to properly use closures in loops?
Quick answer: Use a function factory.
for (var i=0; i<10; i++) {
document.getElementById(i).onclick = (function(x){
return function(){
alert(x);
}
})(i);
}
or the more easily readable version:
function generateMyHandler (x) {
return function(){
alert(x);
}
}
for (var i=0; i<10; i++) {
document.getElementById(i).onclick = generateMyHandler(i);
}
This often confuse people who are new to javascript or functional programming. It is a result of misunderstanding what closures are.
A closure does not merely pass the value of a variable or even a reference to the variable. A closure captures the variable itself! The following bit of code illustrates this:
var message = 'Hello!';
document.getElementById('foo').onclick = function(){alert(message)};
message = 'Goodbye!';
Clicking the element 'foo' will generate an alert box with the message: "Goodbye!". Because of this, using a simple closure in a loop will end up with all closures sharing the same variable and that variable will contain the last value assigned to it in the loop. For example:
for (var i=0; i<10; i++) {
document.getElementById('something'+i).onclick = function(){alert(i)};
}
All elements when clicked will generate an alert box with the number 10. In fact, if we now do i="hello"; all elements will now generate a "hello" alert! The variable i is shared across ten functions PLUS the current function/scope/context. Think of it as a sort of private global variable that only the functions involved can see.
What we want is an instance of that variable or at least a simple reference to the variable instead of the variable itself. Fortunately javascript already has a mechanism for passing a reference (for objects) or value (for strings and numbers): function arguments!
When a function is called in javascript the arguments to that function is passed by reference if it is an object or by value if it is a string or number. This is enough to break variable sharing in closures.
So:
for (var i=0; i<10; i++) {
document.getElementById(i).onclick =
(function(x){ /* we use this function expression simply as a factory
to return the function we really want to use: */
/* we want to return a function reference
so we write a function expression*/
return function(){
alert(x); /* x here refers to the argument of the factory function
captured by the 'inner' closure */
}
/* The brace operators (..) evaluates an expression, in this case this
function expression which yields a function reference. */
})(i) /* The function reference generated is then immediately called()
where the variable i is passed */
}

I've been programming in JavaScript for a long time, and "closure in a loop" is a very broad topic. I assume you are talking about the practice of using (function(param) { return function(){ ... }; })(param); inside of a for loop in order to preserve the "current value" of the loop when that inner function later executes...
The code:
for(var i=0; i<4; i++) {
setTimeout(
// argument #1 to setTimeout is a function.
// this "outer function" is immediately executed, with `i` as its parameter
(function(x) {
// the "outer function" returns an "inner function" which now has x=i at the
// time the "outer function" was called
return function() {
console.log("i=="+i+", x=="+x);
};
})(i) // execute the "closure" immediately, x=i, returns a "callback" function
// finishing up arguments to setTimeout
, i*100);
}
Output:
i==4, x==0
i==4, x==1
i==4, x==2
i==4, x==3
As you can see by the output, all of the inner callback functions all point to the same i, however, since each had its own 'closure', the value of x is actually stored as whatever i was at the time of the outer function's execution.
Commonly when you see this pattern, you would use the same variable name as the parameter and the argument to the outer function: (function(i){ })(i) for instance. Any code inside that function (even if executed later, like a callback function) is going to refer to i at the time you called the "outer function".

Well, the "problem" with closures in such a case is, that any access to i would reference the same variable. That is because of ECMA-/Javascripts function scope or lexical scope.
So to avoid that every call to alert(i); would display a 5 (because after the loop finished i === 5), you need to create a new function which invokes itself at runtime.
To achieve this, you need to create a new function, plus you need the extra paranthesis at the end, to invoke the outer function immediately, so link.onclick has now the returned function as reference.

A closure is a construct in which you reference a variable outside the scope in which it's defined. You usually talk about closures in the context of a function.
var helloFunction;
var finished = false;
while (!finished) {
var message = 'Hello, World!';
helloFunction = function() {
alert(message);
}
finished = true;
}
helloFunction();
Here, I define the variable message, and define a function that references message. When I define the function to use message, I am creating a closure. This means helloFunction holds a reference to message, so that I can continue to use message, even outside of the scope (the loop body) where message is defined.
Addendum
The (i) in parenthesis is a function call. What's happening is:
You define some function(num) {}. This is called an anonymous function, because it's defined inline and doesn't have a name.
function(num) takes an integer argument, and returns a reference to another function, which is defined as alert(num)
The outer anonymous function is immediately called, with the argument i. So num=i. The result of this call is a function which will do alert(i).
The end result is more or less equivalent to: link.onclick = function() { alert(i); };

To answer the last part of your questions. The two parenthesis invoke the function as any other functions. Why you do it here is that you want to keep what the variable "i" is just at that time. So what it does is, invoke the function, the i is sent as a argument "num". Since it's invoke it will remember the value nume in variable links own scoop.
If you did't to this all link click would result in an alert saying "5"
John Resig, founder of jQuery, has a really nice online presentation explaining this. http://ejohn.org/apps/learn/
..fredrik

Assigning a variable to itself in a function

I'm trying to assign a variable to itself in a function, and if the variable has the same name as an argument to the function it doesn't seem to work, but does if they're not the same name. A code example shows this more clearly.
Is this behavior I should expect? This is a pared down example for my d3 use case which prompted this question. I've shown that below as well.
Non working example
var a;
function assign(a) {
a = a;
}
assign("test")
console.log(a)
undefined
Working Example
var a;
function assign(b) {
a = b;
}
assign("test")
console.log(a)
test
Use case
var data
d3.csv("data.csv", function(error, data) {
//Doesn't work for me
data = data
}
console.log(data)
undefined

In your first example, the argument a that is passed to the function shadows the variable a which is defined outside, so: a=a is assignment of the argument (that was passed to the function) to itself.

In Javascript the scope is functional level scope, so whenever the variable is referenced it is searched for its declaration in the containing scope(function), if it finds it it uses it else it keep searching in prototypical chain upto global scope. So in your case it tries to search a and it founds it as argument a so it stops the search there & uses a from argument.
So to avoid the conflict you have use two ways.
Use the different names
If you want to use the same name then use the explicit scope resolution.
Ex.
var a;
function assign(a) {
Global.a = a //Global is placeholder here for outerscope that variable a is coming from.
}
assign("test")
console.log(a);
Useful links for more clear understanding
Closures
Variable Hoisting

you could use the window object to access the global variable.
var a;
function assign(a) {
window.a = a; // specifying the scope.
};
assign("test")
console.log(a)
More information on 15-common-javascript-gotchas

why javascript use call by reference , sometimes :D [duplicate]

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);

Why Self-Executing Anonymous Functions

Today I came a cross the self executing functions, than somehow I ended up knowing about
Self-Executing Anonymous Functions, then I've read this article: http://briancrescimanno.com/how-self-executing-anonymous-functions-work/
The thing is that I don't know WHY to use Self-Executing Anonymous Functions because if I need to do something like:
var test = "a";
(function(foo) {
alert(foo);
})(test);
I could just make something like:
var test = "a";
alert(foo);
Or did I miss anything?
also this can be done to any code inside the function, but I used alert() to make simple
Update:
Even thought I've already accepted and answer I would like to share something I've found, if anyone came across this question later :)
Using this notation we can also make an endless loop like following:
(function loop(){
// do something here
loop();
)();

There are a couple of reasons why one would use an IIFE:
1) No littering
var a = 'foo';
alert(a);
vs
(function() {
var a = 'foo';
alert(a);
}())
Both examples do the same thing, but in the second example there is no a variable inside the outer scope.
2) State capturing
var a = 'foo';
window.setTimeout(function() { alert(a); }, 1);
a = 'bar';
vs
var a = 'foo';
window.setTimeout( (function(a_copy) {
return function() { alert(a_copy); }
}(a)), 1);
a = 'bar';
The first example alerts bar, while the second alerts foo. You will find this technique used especially with loops.

Your initial example isn't worth to be executed in an anonymous function, so its a bad example to understand WHY to use this technique. Here is a good example to explore state capturing:
var list = [{id: 1, data: null}, ...];
for (var i = 0; i < list.length; i++) {
(function(item) {
// start async request, for each item in the list, in the order of the list
asyncAjax("get-data-from-somewhere.json?what=" + list[i].id, function (response) {
// thats the on success callback, which gets executed when the server responded
// each item will have different response times, therefore the order of response is various
// if we would use simply list[i].data, it wouldn't work correctly, because "i" would has the value of list.length, because the iteration has been finished yet.
item.data = response;
});
})(list[i]); // the function will preserve the reference to the list item inside, until the function has been fully executed
}
When writing sync. code, you can always fallback to classic object oriented style of structering your code. So you can avoid closures / instant-anonymous function calls. But as soon as you use async. mechanics, closures get very handy and make your code looking more clean, off course only if you can read and understand closures :)
By the way, you could also simply write:
function(private) {}(outer)
is the same as
(function(private) {})(outer)
but the second is better, because its simply more obvious for the reader.

The syntax you describe is commonly referred to as an "immediately invoked function expression", or IIFE.
One of the common use cases is to emulate private variables:
var ns = (function () {
var x = 1; // "private"
return {
getX: function () {
return x;
}
}
}());
ns.getX(); // 1
ns.x; // undefined because x is "private"
In that example the x variable is local to the IIFE. It's not directly accessible outside of it. However, since it is referred to by the getX method, which is accessible outside of the IIFE (because it's part of the returned object) a reference to x is kept alive. This is what is usually meant by the term "closure".

Self executing functions are not really useful if you just do an alert inside.
Consider something like this:
(function(foo) {
var a = ..
// do something with a and foo
})(test);
The advantage here is that a is "private" inside the method and cannot be used outside the method. So a doesn't end up as a global variable and can't be overwritten by some other piece of javascript which uses a variable of the same name.

Javascript function question

I searched but couldn't find an answer to this seemingly easy question, so...
Suppose I have a loop in which I need to set callbacks. My callback function looks like this:
function callback(var1) { // code }
Now my loop is something like this:
for( //condition)
{
var x = something_different_each_time;
document.getElementById('foo').addEventListener('click', function() { callback(x); }, false);
}
Now it looks like even if the loop runs n times, the anonymous function is compiled only once -- and hence every invocation of callback is called with the same argument (even though x varies in the loop every time).
I must be missing something here.. any help is greatly appreciated! :)

The problem is that the block of the for statement doesn't creates a new scope, for that, the x variable belongs to its enclosing scope, and all anonymous functions refer to the same variable...
Use another function to create a new lexical environment to hold the value of x on each iteration:
for(/*condition*/) {
var x = something_different_each_time;
document.getElementById('foo').addEventListener('click', function () {
return function(y) {
callback(y);
};
}(x), false);
}

See Creating closures in loops: A common mistake
and related questions:
JavaScript closure inside loops – simple practical example
Doesn't JavaScript support closures with local variables?
Access outside variable in loop from Javascript closure
How do JavaScript closures work?

You should calculate x before calling your callback functin!
for( //condition)
{
//var x = something_different_each_time;
document.getElementById('foo').addEventListener('click', function() {
var x = something_different_each_time;
callback(x); }, false);
}

Yes, the x will refer to the same variable in the enclosing scope, and since the function is executing later, it'll have the last value of x. Try this:
.addEventListener(
'click',
(function (i) {
return function () { callback(i); }
})(x),
false
);
This creates a closure with the current value of x locked inside.