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Javascript closure access with callbacks inside loop

what shall I do to make the last row of code return a value?
$scope.runActionwithObjects = function() {
for (var i = 0; i < $scope.Objects.length; i++) {
console.log($scope.Objects[i]); //$scope is accessible
$http.get($scope.Objects[i]["Commit"]).success(function (data) {
console.log($scope.Objects[i]);//return undefined

The problem is due to asynchrony of ajax requests.
When the success callback is executed, your loop has already finished and the i variable is already equal to $scope.Objects.length.
Try forEach. This function will create different closures for items in the array.
$scope.Objects.forEach(function(currentObject){
console.log(currentObject); //$scope is accessible
$http.get(currentObject["Commit"]).success(function (data) {
console.log(currentObject);
});
});

The reason your $scope.Objects[i] is undefined because the variable i is always = $scope.Objects.lenth + 1, for example you got 5 elements, the i will be 6, because the at the time of callback, it already got the last value.
One solution is to bind needed object to that method, so we can access it via this(we can not reference directly by closure to ref variable, because it's still stored the last item), for example:
for (var i = 0; i < $scope.Objects.length; i++) {
var ref = $scope.Objects[i];
// console.log($scope.Objects[i]); //$scope is accessible
var successCallback = (function (data) {
console.log(this);//return the ref
}).bind(ref);
$http.get('').success(successCallback);
}
}

Call function from an array asynchronously in JavaScript [duplicate]

This question already has an answer here:
Why is the loop assigning a reference of the last index element to? [duplicate]
(1 answer)
Closed 8 years ago.
I've created a simple observer model in a JavaScript WebApp to handle event-listeners on a more complex JS-Object model (no DOM events). One can register event listener functions that are then stored in an array. By calling a member function out of the wider application of the model the event listeners are executed. So far so good. Here's the implementation that works well:
var ModelObserver = function() {
this.locationObserverList = [];
}
ModelObserver.prototype.emitEvent = function(eventtype, data) {
for(var i=0; i < this.locationObserverList.length; i++) {
var fns = this.locationObserverList[i];
fns(data); // function is being called
}
};
ModelObserver.prototype.registerLocationListener = function( fn) {
this.locationObserverList.push(fn);
};
If tested it with two listeners in a small sample html site, all good.
Now I want to make the call to the function asynchronously. I tried to change the code of the respective function as follows:
ModelObserver.prototype.emitEvent = function(eventtype, data) {
for(var i=0; i < this.locationObserverList.length; i++) {
var fns = this.locationObserverList[i];
setTimeout(function() {fns(data);}, 0);
}
};
Unfortunately I have a problem here: only the second listener is being called, but now twice. It seems to be a conflict with the fns variable, so I tried this:
ModelObserver.prototype.emitEvent = function(eventtype, data) {
var fns = this.locationObserverList;
for(var i=0; i < this.locationObserverList.length; i++) {
setTimeout(function() {fns[i](data);}, 0);
}
};
Now I get an error: "Uncaught TypeError: Property '2' of object [object Array] is not a function".
Does anyone have an idea how to get this working asynchronously?

The anonymous function you're giving setTimeout has an enduring reference to the variables it closes over, not a copy of them as of when it was created.
You need to make it close over something else. Usually, you use a function that builds the function for setTimeout and closes over args to the builder:
ModelObserver.prototype.emitEvent = function(eventtype, data) {
for(var i=0; i < this.locationObserverList.length; i++) {
var fns = this.locationObserverList[i];
setTimeout(buildHandler(fns, data), 0);
// Or combining those two lines:
//setTimeout(buildHandler(this.locationObserverList[i], data), 0);
}
};
function buildHandler(func, arg) {
return function() {
func(arg);
};
}
There, we call buildHandler with a reference to the function and the argument we want it to receive, and buildHandler returns a function that, when called, will call that function with that argument. We pass that returned function into setTimeout.
You can also do this with ES5's Function#bind, if you're in an ES5 environment (or include an appropriate shim, as this is shimmable):
ModelObserver.prototype.emitEvent = function(eventtype, data) {
for(var i=0; i < this.locationObserverList.length; i++) {
var fns = this.locationObserverList[i];
setTimeout(fns.bind(undefined, data), 0);
// Or combining those two lines:
//setTimeout(this.locationObserverList[i].bind(undefined, data), 0);
}
};
Skipping some details, that basically does what buildHandler above does.
More on this (on my blog): Closures are not complicated
Side note: By scheduling these functions to be called later via setTimeout, I don't think you can rely on them being called in order. That is, even if you schedule 1, 2, and 3, I don't know that you can rely on them being called that way. The (newish) spec for this refers to a "list" of timers, suggesting order, and so one might be tempted to think that registering timers in a particular order with the same timeout would have them execute in that order. But I don't (skimming) see anything in the spec guaranteeing that, so I wouldn't want to rely on it. A very quick and dirty test suggested the implementations I tried it on did that, but it's not something I'd rely on.

ModelObserver.prototype.emitEvent = function(eventtype, data) {
var fns = this.locationObserverList;
for(var i=0; i < this.locationObserverList.length; i++) {
(function(j){
setTimeout(function() {fns[i](data);}, 0);
}(i));
}
};
Try this

The second try is not going to work. In your first sample try -
setTimeout(function() {this.fns(data);}, 0);

how can I get the value of a variable in a closure in javascript [duplicate]

This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 8 years ago.
I'm creating an array of callbacks like this:
function createFunctions(n) {
var callbacks = [];
for (var i=0; i<n; i++) {
callbacks.push(function() {
return i;
});
}
return callbacks;
}
And, here's the test that I'm working with:
var callbacks = createFunctions(5);
for (var i=0; i<callbacks.length; i++) {
Test.assertEquals(callbacks[i](), i, 'Function with index ' + i);
}
Basically, I want the callback to return it's index in the array (i.e. callback[1]() should return 1). But, i defined in createFunctions is set to 6 when the test runs so they always return 6. I've tried creating a local variable to hold the value in the anonymous function but that's not working either.
Any ideas how to make this work?

A closure has an enduring reference to the variables it closes over, not a copy of their value as of when it was created. That's why all of your functions see 6: That's the value that i has as of when those functions are called.
If you need them to see different values, make the closures close over something else that doesn't change. Here's one way:
function createFunctions(n) {
var callbacks = [];
for (var i=0; i<n; i++) {
callbacks.push(makeCallback(i));
}
return callbacks;
function makeCallback(index) {
return function() {
return index;
};
}
}
Or:
function createFunctions(n) {
var callbacks = [];
for (var i=0; i<n; i++) {
callbacks.push(makeCallback(i));
}
return callbacks;
}
function makeCallback(index) {
return function() {
return index;
};
}
(In this example, it doesn't matter whether makeCallback is within createFunctions or outside it.)
Now, the callbacks close over the context containing the index argument. They each get their own copy of that context (which is created by calling the makeCallback function), and so since nothing ever changes index, its value remains unchanged.
More (on my blog): Closures are not complicated
This feature of closures closing over the context of the variable is very, very useful. Consider:
function allocator(start) {
return function() {
return ++start;
};
}
var f = allocator(0);
alert(f()); // "1"
alert(f()); // "2"
alert(f()); // "3"
That wouldn't work if the function created by allocator had a copy of the value of start. It works because the function created by allocator has a reference (through the context) to the variable itself.

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 scope and calling a function

My code:
for (var i = 0; i < mapInfos.length; i++) {
var x = function () { doStuff(i); };
google.maps.event.addListenerOnce(mapInfos[i].map, 'tilesloaded', x);
}
The doStuff method simply alerts the value of i. mapInfos has two entries, so you'd expect it to alert 0 and 1, but instead it alerts 2 and 2. I can appreciate vaguely why it is doing this (although var i should keep it local to the scope of the loop?) but how can I make it work as intended?

edit — note that when first posted, the original question included a link to a jsfiddle that seemed to be a relevant example of what the current question is trying to achieve, only it appears to work ...
The code in the jsfiddle works because there's only one "i" in that code. The "i" used in the second loop (where the functions are actually called) is the same "i" as used in the first loop. Thus, you get the right answer because that second loop is running "i" through all the values from zero through four again. If you added:
i = 100;
functions[0]();
you'd get 100 printed out.
The only way to introduce a new scope in JavaScript is a function. One approach is to write a separate "function maker" function:
function makeCallback(param) {
return function() {
doStuff(param);
};
}
Then in your loop:
for (var i = 0; i < mapInfos.length; i++) {
var x = makeCallback(i);
google.maps.event.addListenerOnce(mapInfos[i].map, 'titlesloaded', x);
}
That'll work because the call to the "makeCallback" function isolates a copy of the value of "i" into a new, unique instance of "param" in the closure returned.

Create a new scope for it.
Functions create scope.
function doStuffFactory(i) {
return function () { doStuff(i); };
}
for (var i = 0; i < mapInfos.length; i++) {
var x = doStuffFactory(i);
google.maps.event.addListenerOnce(mapInfos[i].map, 'tilesloaded', x);
}

Change it to
var x = function (param) { doStuff(param); };
Obviously what is going on is that you are alerting a variable that is changing. With the above change it copies it so even if i changes it will still alert the right value.

Javascript doesn't have block scope, so you don't get an x that's local to the loop. Yea!
It has function scope, though.

Yep, weird isn't it!Pointy has an explanation
I have no idea why your first example worked (I wasn't expecting it to) Pointy has an explanation of why your first example worked - The reason why your second one doesn't is because i is scoped to the function containing the for loop, not to the scope defined by the for loop. In fact the only things that have scope in JavaScript are functions. This means that by the time your function gets executed i is 2.
What you need to do is create a scope, for example:
for (var i = 0; i < mapInfos.length; i++) {
var x = (function() {
return function () { doStuff(i); };
})(i);
google.maps.event.addListenerOnce(mapInfos[i].map, 'tilesloaded', x);
}
See JavaScript closures in for-loops for more.