I am looking at a javascript code that manipulates an HTML A tag , and I'm having trouble understanding how it sets up the "onclick" property. It seems to be telling it to update ytplayer_playitem with the index variable j and then call ytplayer_playlazy(1000)
But what's up with all the parentheses? What details in the javascript syntax allows it to be setup like this?
var a = document.createElement("a");
a.href = "#ytplayer";
a.onclick = (function (j) {
return function () {
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
})(i);
Well, basically, the value of onclick is a function that will get called when the element is clicked. Whatever you want to happen when the user clicks the element goes in the body of the function.
You could create a named function and then assign it to the element's onclick attribute:
function youClickedMe() {
...
}
a.onclick = youClickedMe
but that clutters up the namespace with a function name that is never referenced anywhere else. It's cleaner to create an anonymous function right where you need it. Normally, that would look like this:
a.onclick = function() { ... }
But if we try that with your specific example:
a.onclick = function() {
ytplayer_playitem = something; // ??
ytplayer_playlazy(1000);
}
We see that it hard-codes the something that gets played. I'm assuming the original code was taken from a loop which generates several clickable links to play; with the code just above, all of those links would play the same thing, which is probably not what you want.
So far, so straightforward, but this next leap is where it gets tricky. The solution seems obvious: if you're in a loop, why not just use the loop variable inside the function body?
// THIS DOESN'T WORK
a.onclick = function() {
ytplayer_playitem = i;
ytplayer_playlazy(1000);
}
That looks like it should work, but unfortunately the i inside the function refers to the value of the variable i when the function is called, not when it's created. By the time the user clicks on the link, the loop that created all the links will be done and i will have its final value - probably either the last item in the list or one greater than that item's index, depending on how the loop is written. Whatever its value is, you once again have the situation where all links play the same item.
The solution in your code gets a little meta, by using a function whose return value is another function. If you pass the loop control variable to the generating function as an argument, the new function it creates can reference that parameter and always get the value that was originally passed in, no matter what has happened to the value of the outer argument variable since:
function generate_onclick(j) {
// no matter when the returned function is called, its "j" will be
// the value passed into this call of "generate_onclick"
return function() { ytplayer_playitem = j; ytplayer_playlazy(1000); }
}
To use that, call it inside the loop like this:
a.onclick = generate_onclick(i);
Each generated function gets its very own j variable, which keeps its value forever instead of changing when i does. So each link plays the right thing; mission accomplished!
That's exactly what your posted original code is doing, with one small difference: just like the first step in my explanation, the author chose to use an anonymous function instead of defining a named one. The other difference here is that they are also calling that anonymous function immediately after defining it. This code:
a.onclick = (function (j) { ... })(i)
is the anonymous version of this code:
function gen(j) { ... }
a.onclick = gen(i)
The extra parens around the anonymous version are needed because of JavaScript's semicolon-insertion rules; function (y) {...}(blah) compiles as a standalone function definition followed by a standalone expression in parentheses, rather than a function call.
"But what's up with all the parentheses? "
Most of the parentheses are just doing what you'd expect.
There's an extra set that isn't technically needed, but is often used as a hint that the function is being invoked.
// v-v---these are part of the function definition like normal
a.onclick = (function (j) {
// ^-----------this and...v
return function () {
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
// v---...this are technically not needed here, but are used as a hint
})(i);
// ^-^---these invoked the function like normal
"What details in the javascript syntax allows it to be setup like this?"
The upshot is that the function is invoked immediately, and passed i so that its value is referenced by the j parameter in the immediately invoked function.
This creates a variable scope that the returned function will continue to have access to. This way it always has access to the j variable, and not the i that gets overwritten in the loop.
These inlined functions are abused a bit IMO. It becomes clearer if you simply make it a named function.
for(var i = 0; i < 10; i++) {
// create the new element
a.onclick = createHandler(i);
// append it somewhere
}
function createHandler (j) {
return function () {
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
}
The resulting handler is exactly the same, but the code is much less cryptic.
Right, I'm going to guess that the surrounding code looks like this:
for (var i = 0; i < playitems.length; i++) {
// above code here
}
Now, you could do the obvious thing here, and assign the onclick property like this:
a.onclick = function() {
ytplayer_playitem = i;
ytplayer_playlazy(1000);
};
However that wouldn't work very well, because the value of i changes. Whichever link was clicked, the last one would be the one activated, because the value of i at that point would be the last one in the list.
So you need to prevent this happening. You need to do this by creating a new scope, which is done by creating an extra function, which is immediately invoked:
(function (j) {
// some code here
})(i);
Because i has been passed into the function, the value is passed rather than a reference to the variable being kept. This means that you can now define a function which will have a reference to the correct value. So you get your extra function to return the click handling function:
a.onclick = (function (j) { // j is the right number and always will be
return function () { // this function is the click handler
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
})(i);
So each a element has its own click handler function, each of which has its own individual j variable, which is the correct number. So the links, when clicked, will perform the function you want them to.
a.onclick = (function (j) {
return function () {
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
})(i);
This creates a "closure" to ensure that the value of i that is bound to the handler is the value of i "at that time" and not i in the general.
In your code, the function inside the () is an expression, executed and passed the variable i. This is the (i) you see in the end part. In this executed function expression, the i becomes the local variable j. This executed function expression returns the handler function that is to be bound the onclick event carrying the value of j which was i "at that time"
if i did not use the closure:
//suppose i is 1
var i = 1;
a.onclick = function () {
ytplayer_playitem = i;
ytplayer_playlazy(1000);
};
//and changed i
i = 2;
//if you clicked the <a>, it would not be 1 onclick but 2 because you
//did not assign the value of i "at that time". i is "tangible" this way
a.onclick = (function (j) {
return function () {
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
})(i);
What you have here is a self-invoking anonymous function. Let's break it down, first replacing the body of the function with something simpler (return j + 1;):
function( j ) { return j + 1; }
This s a run-of-the-mill anonymous function or closure. This line of code is an expression, and so it has a value, and that value is a function. Now we could do this:
var foo = function( j ) { return j + 1; }
foo( 5 ); // => 6
You recognize this, I'm sure—we're assigning the anonymous function to the variable foo, and then calling the function by name with the argument i. But, instead of creating a new variable, because the closure is an expression we can call it like this instead:
( function( j ) { return j + 1; } )( 5 ); // => 6
Same result. Now, it's just returning j + 1 but in your code it returns something else: Another anonymous function:
return function() { /* ... */ }
What happens when we have a self-invoking anonymous function that returns a function? The result is the "inner" function that was returned:
a.onclick = ( function( j ) {
return function() {
ytplayer_playitem = j;
ytplayer_playlazy( 1000 );
}
}
)( i );
If i was equal to 9 then a.onclick would now hold a function equivalent to this:
function() {
ytplayer_playitem = 9;
ytplayer_playlazy( 1000 );
}
As others have pointed out, the usefulness of this is that when ( function( j ) { /* ... */ } )( i ) is invoked you are capturing the value of i at that time and putting it into j rather than creating a reference to the value i holds, which may (and probably will) change later on.
Related
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 8 years ago.
I've got the following code snippet.
function addLinks () {
for (var i=0, link; i<5; i++) {
link = document.createElement("a");
link.innerHTML = "Link " + i;
link.onclick = function () {
alert(i);
};
document.body.appendChild(link);
}
}
The above code is for generating 5 links and binding each link with an alert event to show the current link id. But It doesn't work. When you click the generated links they all say "link 5".
But the following code snippet works as our expectation.
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);
}
}
The above 2 snippets are quoted from here. As the author's explanation seems the closure makes the magic.
But how it works and How closure makes it work are all beyond my understanding. Why the first one doesn't work while the second one works? Can anyone give a detailed explanation about the magic?
Quoting myself for an explanation of the first example:
JavaScript's scopes are function-level, not block-level, and creating a closure just means that the enclosing scope gets added to the lexical environment of the enclosed function.
After the loop terminates, the function-level variable i has the value 5, and that's what the inner function 'sees'.
In the second example, for each iteration step the outer function literal will evaluate to a new function object with its own scope and local variable num, whose value is set to the current value of i. As num is never modified, it will stay constant over the lifetime of the closure: The next iteration step doesn't overwrite the old value as the function objects are independant.
Keep in mind that this approach is rather inefficient as two new function objects have to be created for each link. This is unnecessary, as they can easily be shared if you use the DOM node for information storage:
function linkListener() {
alert(this.i);
}
function addLinks () {
for(var i = 0; i < 5; ++i) {
var link = document.createElement('a');
link.appendChild(document.createTextNode('Link ' + i));
link.i = i;
link.onclick = linkListener;
document.body.appendChild(link);
}
}
We have 5 divs on the page, each with an ID ... div1, div2, div3, div4, div5
jQuery can do this ...
for (var i=1; i<=5; i++) {
$("#div" + i).click ( function() { alert ($(this).index()) } )
}
But really addressing the problem (and building this up slowly) ...
STEP 1
for (var i=1; i<=5; i++) {
$("#div" + i).click (
// TODO: Write function to handle click event
)
}
STEP 2
for (var i=1; i<=5; i++) {
$("#div" + i).click (
function(num) {
// A functions variable values are set WHEN THE FUNCTION IS CALLED!
// PLEASE UNDERSTAND THIS AND YOU ARE HOME AND DRY (took me 2 years)!
// Now the click event is expecting a function as a handler so return it
return function() { alert (num) }
}(i) // We call the function here, passing in i
)
}
SIMPLE TO UNDERSTAND ALTERNATIVE
If you can't get your head around that then this should be easier to understand and has the same effect ...
for (var i=1; i<=5; i++) {
function clickHandler(num) {
$("#div" + i).click (
function() { alert (num) }
)
}
clickHandler(i);
}
This should be simple to understand if you remember that a functions variable values are set when the function is called (but this uses the exact same thought process as before)
Basically, in the first example you're binding the i inside the onclick handler directly to the i outside the onclick handler. So when the i outside the onclick handler changes, the i inside the onclick handler changes too.
In the second example, instead of binding it to the num in the onclick handler, you're passing it into a function, which then binds it to the num in the onclick handler. When you pass it into the function, the value of i is copied, not bound to num. So when i changes, num stays the same. The copy occurs because functions in JavaScript are "closures", meaning that once something is passed into the function, it's "closed" for outside modification.
Others have explained what's going on, here's an alternative solution.
function addLinks () {
for (var i = 0, link; i < 5; i++) {
link = document.createElement("a");
link.innerHTML = "Link " + i;
with ({ n: i }) {
link.onclick = function() {
alert(n);
};
}
document.body.appendChild(link);
}
}
Basically the poor mans let-binding.
In the first example, you simply bind this function to the onclick event:
function() {alert(i);};
This means that on the click event js should alert the value of the addlink functions i variable. Its value will be 5 because of the for loop().
In the second example you generate a function to be bound with another function:
function (num) {
return function () { alert(num); };
}
This means: if called with a value, return me a function that will alert the input value. E.g. calling function(3) will return function() { alert(3) };.
You call this function with the value i at every iteration, thus you create separate onclick functions for each links.
The point is that in the first example your function contained a variable reference, while in the second one with the help of the outer function you substituted the reference with an actual value. This is called a closure roughly because you "enclose" the current value of a variable within your function instead of keeping a reference to it.
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 8 years ago.
I've got the following code snippet.
function addLinks () {
for (var i=0, link; i<5; i++) {
link = document.createElement("a");
link.innerHTML = "Link " + i;
link.onclick = function () {
alert(i);
};
document.body.appendChild(link);
}
}
The above code is for generating 5 links and binding each link with an alert event to show the current link id. But It doesn't work. When you click the generated links they all say "link 5".
But the following code snippet works as our expectation.
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);
}
}
The above 2 snippets are quoted from here. As the author's explanation seems the closure makes the magic.
But how it works and How closure makes it work are all beyond my understanding. Why the first one doesn't work while the second one works? Can anyone give a detailed explanation about the magic?
Quoting myself for an explanation of the first example:
JavaScript's scopes are function-level, not block-level, and creating a closure just means that the enclosing scope gets added to the lexical environment of the enclosed function.
After the loop terminates, the function-level variable i has the value 5, and that's what the inner function 'sees'.
In the second example, for each iteration step the outer function literal will evaluate to a new function object with its own scope and local variable num, whose value is set to the current value of i. As num is never modified, it will stay constant over the lifetime of the closure: The next iteration step doesn't overwrite the old value as the function objects are independant.
Keep in mind that this approach is rather inefficient as two new function objects have to be created for each link. This is unnecessary, as they can easily be shared if you use the DOM node for information storage:
function linkListener() {
alert(this.i);
}
function addLinks () {
for(var i = 0; i < 5; ++i) {
var link = document.createElement('a');
link.appendChild(document.createTextNode('Link ' + i));
link.i = i;
link.onclick = linkListener;
document.body.appendChild(link);
}
}
We have 5 divs on the page, each with an ID ... div1, div2, div3, div4, div5
jQuery can do this ...
for (var i=1; i<=5; i++) {
$("#div" + i).click ( function() { alert ($(this).index()) } )
}
But really addressing the problem (and building this up slowly) ...
STEP 1
for (var i=1; i<=5; i++) {
$("#div" + i).click (
// TODO: Write function to handle click event
)
}
STEP 2
for (var i=1; i<=5; i++) {
$("#div" + i).click (
function(num) {
// A functions variable values are set WHEN THE FUNCTION IS CALLED!
// PLEASE UNDERSTAND THIS AND YOU ARE HOME AND DRY (took me 2 years)!
// Now the click event is expecting a function as a handler so return it
return function() { alert (num) }
}(i) // We call the function here, passing in i
)
}
SIMPLE TO UNDERSTAND ALTERNATIVE
If you can't get your head around that then this should be easier to understand and has the same effect ...
for (var i=1; i<=5; i++) {
function clickHandler(num) {
$("#div" + i).click (
function() { alert (num) }
)
}
clickHandler(i);
}
This should be simple to understand if you remember that a functions variable values are set when the function is called (but this uses the exact same thought process as before)
Basically, in the first example you're binding the i inside the onclick handler directly to the i outside the onclick handler. So when the i outside the onclick handler changes, the i inside the onclick handler changes too.
In the second example, instead of binding it to the num in the onclick handler, you're passing it into a function, which then binds it to the num in the onclick handler. When you pass it into the function, the value of i is copied, not bound to num. So when i changes, num stays the same. The copy occurs because functions in JavaScript are "closures", meaning that once something is passed into the function, it's "closed" for outside modification.
Others have explained what's going on, here's an alternative solution.
function addLinks () {
for (var i = 0, link; i < 5; i++) {
link = document.createElement("a");
link.innerHTML = "Link " + i;
with ({ n: i }) {
link.onclick = function() {
alert(n);
};
}
document.body.appendChild(link);
}
}
Basically the poor mans let-binding.
In the first example, you simply bind this function to the onclick event:
function() {alert(i);};
This means that on the click event js should alert the value of the addlink functions i variable. Its value will be 5 because of the for loop().
In the second example you generate a function to be bound with another function:
function (num) {
return function () { alert(num); };
}
This means: if called with a value, return me a function that will alert the input value. E.g. calling function(3) will return function() { alert(3) };.
You call this function with the value i at every iteration, thus you create separate onclick functions for each links.
The point is that in the first example your function contained a variable reference, while in the second one with the help of the outer function you substituted the reference with an actual value. This is called a closure roughly because you "enclose" the current value of a variable within your function instead of keeping a reference to it.
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);
I have the following code that adds an onmouseover event to a bullet onload
for (var i = 0; i <= 3; i++) {
document.getElementById('menu').getElementsByTagName('li')[i].onmouseover = function () { addBarOnHover(i); };
}
This is the function that it is calling. It is supposed to add a css class to the menu item as the mouse goes over it.
function addBarOnHover(node) {
document.getElementById('menu').getElementsByTagName('li')[node].className = "current_page_item"; }
When the function is called, I keep getting the error:
"document.getElementById("menu").getElementsByTagName("li")[node] is
undefined"
The thing that is stumping me is I added an alert(node) statement to the addBarOnHover function to see what the value of the parameter was. The alert said the value of the parameter being passed was 4. I'm not sure how this could happen with the loop I have set up.
Any help would be much appreciated.
This is a common problem when you close over an iteration variable. Wrap the for body in an extra method to capture the value of the iteration variable:
for (var i = 0; i <= 3; i++) {
(function(i){ //here
document.getElementById('menu').getElementsByTagName('li')[i].onmouseover = function () { addBarOnHover(i); };
})(i); //here
}
an anonymous function is created each time the loop is entered, and it is passed the current value of the iteration variable. i inside the anonymous function refers to the argument of this function, rather than the i in the outer scope.
You could also rename the inner variable for clarity:
for(var i=0; i<=3; i++){
(function(ii){
//use ii as i
})(i)
}
Without capturing the iteration variable, the value of i when it is finally used in the anonymous handler has been already changed to 4. There's only one i in the outer scope, shared between all instances of the handler. If you capture the value by an anonymous function, then the argument to that function is used instead.
i is being passed as a reference (not by value), so once the onmouseover callback is called, the value of i has already become 4.
You'll have to create your callback function using another function:
var menu = document.getElementById('menu');
var items = menu.getElementsByTagName('li');
for (var i = 0; i <= 3; i++) {
items[i].onmouseover = (function(i) {
return function() {
addBarOnHover(i);
};
})(i);
}
You could make it a little more readable by making a helper function:
var createCallback = function(i) {
return function() {
addBarOnHover(i);
};
};
for (var i = 0; i <= 3; i++) {
items[i].onmouseover = createCallback(i);
}
I know that this code doesn't work and I also know why.
However, I do not know how to fix it:
JavaScript:
var $ = function(id) { return document.getElementById(id); };
document.addEventListener('DOMContentLoaded', function()
{
for(var i = 1; i <= 3; i++)
{
$('a' + i).addEventListener('click', function()
{
console.log(i);
});
}
});
HTML:
1
2
3
I want it to print the number of the link you clicked, not just "4".
I will prefer to avoid using the attributes of the node (id or content), but rather fix the loop.
Wrap the loop block in its own anonymous function:
document.addEventListener('DOMContentLoaded', function()
{
for(var i = 1; i <= 3; i++)
{
(function(i) {
$('a' + i).addEventListener('click', function() {
console.log(i);
})
})(i);
}
}
This creates a new instance of i that's local to the inner function on each invocation/iteration. Without this local copy, each function passed to addEventListener (on each iteration) closes over a reference to the same variable, whose value is equal to 4 by the time any of those callbacks execute.
The problem is that the inner function is creating a closure over i. This means, essentially, that the function isn't just remembering the value of i when you set the handler, but rather the variable i itself; it's keeping a live reference to i.
You have to break the closure by passing i to a function, since that will cause a copy of i to be made.
A common way to do this is with an anonymous function that gets immediately executed.
for(var i = 1; i <= 3; i++)
{
$('a' + i).addEventListener('click', (function(localI)
{
return function() { console.log(localI); };
})(i);
}
Since you're already using jQuery, I'll mention that jQuery provides a data function that can be used to simplify code like this:
for(var i = 1; i <= 3; i++)
{
$('a' + i).data("i", i).click(function()
{
console.log($(this).data("i"));
});
}
Here, instead of breaking the closure by passing i to an anonymous function, you're breaking it by passing i into jQuery's data function.
The closure captures a reference to the variable, not a copy, which is why they all result in the last value of the 'i'.
If you want to capture a copy then you will need to wrap it in yet another function.