I am trying to set up an on-click callback for an HTML that causes another node to become visible. Along the way, I was surprised to find out that the following two statements are not equivalent:
$("#title").click($("#content").toggle);
$("#title").click(function() {
$("#content").toggle();
}
The first statement ultimately results in a TypeError when the element is finally clicked, with a message of "undefined is not a function," which I surmised to indicate that whatever I was assigned to the onclick callback ended up being undefined and somehow doesn't persist in memory.
The workaround is simple (just use the statement of the second form), but what I really want to understand is why passing the toggle function as an object doesn't work when it finally gets called. I can see that the two are semantically different: the first executes the $("#content") call when binding the event and the other executes it when the event occurs, but I don't understand why that should matter.
In case it is relevant to the answer, the code in question is located inside of a function (that has presumably returned by the time the user can click anything).
In the first example, you're passing the toggle function for jQuery to execute as the event handler.
However, when jQuery executes the event handler, it sets the value of this to be the DOM element the event fired on.
toggle, however, expects it to be the jQuery object (which it would be if called normally), as it uses this.animate() in its implementation.
This is why you see "undefined is not a function", as this.animate is "undefined", and you're trying to call it as a function.
It's important to appreciate the resolution of this inside a function is deferred until the function is executed. This means a single function can see a different this value between invocations. The value of this can be altered using bind(), new, call(), apply() or by referencing the object differently; for more info see here, or How does the "this" keyword work?
The jQuery function, as in this -> $(), is just a function, think of it as
var $ = function(selector, context) {
// do stuff with selector etc
}
That's really simplified, but when you're calling the jQuery function (as in $()) with a valid selector, it gets the DOM node and returns something like this.
[
0 : <div id="title"></div>,
context : document,
selector : "#title",
jquery : "1.11.0",
.....
etc
]
this is the array-like object jQuery returns, and as you can see 0 is the native DOM node, and it's the reason we can do $('#title')[0] to get the native DOM node.
There is however something that one really can't see from a simple console.log, and that's the methods that are prototyped onto that array-like object, we could however use a for..in loop to see them in the console.
var title = $('#title');
for (var key in title)
console.log(key)
FIDDLE
This would return a long list of all the prototyped and non-prototyped methods available on this object
get
each
map
first
last
eq
extend
find
filter
not
is
has
closest
....
etc
Notice that these are all the jQuery methods added to the $() function with $.prototype, but jQuery uses a shorter name, $.fn, but it does the same thing.
So all the jQuery functions we know are added to the main $() function as properties, and the new keyword is used internally to return a new instance of the $() function with those prototyped properties, and that's why we can use dot notation, or for that matter bracket notation and chain on methods to the $() function, like this
$().find()
// or
$()[find]()
When objects are extended with prototyped properties like this, the value of this is also set inside the methods, so now that we understand a little bit about how it works, we can recreate a really simple jQuery version
var $ = function(selector, context) {
if (this instanceof $) {
this.context = context || document;
this[0] = this.context.querySelector(selector);
return this;
}else{
return new $(selector, context);
}
}
This is simplified a lot from how jQuery works, but in principle it's the same, when $() is called, it checks if it's an instance of itself, otherwise it creates a new instance with the new keyword and calls itself again as a new instance.
When it is a new instance, it gets the element and the other properties it needs, and returns those.
If we were to prototype on a method to that instance, we could chain it like jQuery does, so lets try that
$.prototype.css = function(style, value) {
this[0].style[style] = value;
}
and now we can do this
$('#title').css('color', 'red');
we've almost created jQuery, only 10000 lines of code to go.
FIDDLE
Notice how we have to use this[0] to get the element, we don't have to do that in jQuery when we use something like click, we can just use this, so how does that work ?
Lets simplify that as well, as it's crucial to understand why the code in the question doesn't work
$.prototype.click = function(callback) {
var element = this[0]; // we still need [0] to get the element
element.addEventListener('click', callback.bind(element), false);
return this;
}
What we did there was use bind() to set the value of this inside the callback function so we don't have to use this[0], we can simply use this.
FIDDLE
Now that's cool, but now we can no longer use any of the other methods we've created and prototyped to the object, as this is no longer the object, it's the DOM node, so this fails
$('#element').click(function() {
this.css('color', 'red'); // error, <div id="element".. has no css()
// however this would work, as we now have the DOM node
this.style.color = 'red';
});
The reason it fails is because we now have the native DOM node, and not the jQuery object.
So finally to answer the question asked.
The reason this works ...
$("#title").click(function() {
$("#content").toggle();
});
... is because you're calling the toggle() function, and the correct value of this is set, in this case it would be the jQuery object containing #content as toggle() has no callback that uses bind(), it simply passes the jQuery object, an object similar to what we can see at the top of this answer
Internally toggle() does
$.prototype.toggle = function() {
this.animate();
}
see how it uses this directly whithout doing anything other than calling another jQuery function, it requires that this is a jQuery object, not a native DOM element.
Lets repeat that, toggle() requires that this inside the function is a jQuery object, it can not be anything other than a jQuery object.
-
Now lets move on back to click again, when you do
$("#title").click(function() {
console.log(this)
});
the console would show the native DOM element, something like <div id="title"></div>
Now we can reference a named function instead
$("#title").click(myClickHandler);
function myClickHandler() {
console.log(this)
});
and the result would be exactly the same, we would get the native DOM element in the console -> <div id="title"></div>, which is not suprising as this is exactly the same as the one above using an anonymous function.
What you're doing is referencing the toggle() function like this
$("#title").click($("#content").toggle);
It's exactly the same as the example above, but now you're referencing toggle(), and when called it will be called with the value of this set to the native DOM element in the click function, it would go like this
$("#title").click($("#content").toggle);
$.prototype.toggle = function() {
console.log(this); // would still be <div id="title"></div>
this.animate(); // fails as <div id="title"></div> has no animate()
}
This is what is happening, toggle() is expecting this to be a jQuery object, but instead it gets the native DOM node for the element in the click handler.
Read that again, this inside the toggle() function would be the native #title element, which isn't even the correct element, as that's how javascript and jQuery works, see the long explanation above for how this is set in the prototyped methods etc.
When you use this in a JS function, it refers to whatever object the function is currently being called on, not where it was defined. For instance, you can define a function and copy it onto another object, like this:
foo = {'name': 'foo'}; bar = {'name': 'bar'};
foo.test= function() { console.log(this.name); }
bar.test= foo.test;
foo.test(); // logs 'foo'
bar.test(); // logs 'bar'
When you run foo.test(), this is set to point at foo; but when you run the same function as bar.test(), this is set to bar instead. There is nothing in the function that knows it was originally part of foo, so you basically have two separate but identical functions, like this:
foo.test = function() { console.log(this.name); }
bar.test = function() { console.log(this.name); }
When you run $("#title").click($("#content").toggle);, a similar thing happens - you get a reference to the toggle function, and copy that function into jQuery's list of event handlers. When the callback runs, the $("#content") part is forgotten, just like the foo was above, so when the implementation in jQuery looks at this to see what you want to toggle, it will find the wrong thing.
Exactly what it finds instead has an extra little quirk: jQuery sets this on click handlers to be the DOM element that was clicked on (there are various ways in JS of explicitly telling a function what it should use as this). The exact error comes about because the implementation of toggle is expecting a jQuery object, not a native DOM object, but even if a jQuery object was set as this, it would be the wrong node: you clicked on $('#title'), but want to toggle $('#content'), and jQuery has no way of knowing that.
For completeness, to explain why $("#title").click(function() { $("#content").toggle(); } does work: here the function being saved in jQuery is an anonymous function, which doesn't make any use of this, so doesn't care what it gets set to when the callback finally fires. When the event runs (when you click) it calls toggle with an explicit context (the object returned by the $('#content') lookup), which is exactly what it's expecting.
Related
I am trying to pass function reference as event handler in jQuery. I would like to use a shorthand like in the simple example below...
$("a").click(console.debug.bind(undefined,this));
...rather than passing explicitly the whole function body:
$("a").click(function() {
console.debug(this)
});
Moreover, I would like to access elements selected by jQuery in my shorthand function (and pass them as a parameter). In other words: I expect to have a result of $("a") as a this (or any other code that will retrieve the result).
So far I've tried:
var a = function() {
console.debug(this);
};
var b = console.debug.bind(undefined,this);
$("a").click(function() {console.debug(this)}); // prints link
$("a").click(a); // prints link
b(); // prints Window
$("a").click(console.debug.bind(undefined,this)); // prints Window & jQuery.Event
Here is the fiddle:
https://jsfiddle.net/hbqw2z93/1/
My questions are:
Is it possible to use such construction and meet all requirements, without definition of additional variables - just one line as shown above?
Is it possible to access jQuery's selection result using described approach?
Why in the given scope this becomes 'merged' Window and jQuery.Event object?
You already using it, aren't you? :) It's limited, but it works in your own fiddle
jQuery will pass event object to your specified function. You can use function bind to pass that as an argument (you already have this working in your fiddle)
It doesn't. See what's happening:
jQuery passed one argument to click handler function - event object. You pass console.debug.bind(undefined, this) as a handler function so jQuery will call it with one argument.
Then, when you are binding you are asking to use 'undefined' as a 'this' object inside the function and sending an extra argument - 'this', which is a Window at this scope because you are binding at the highest level.
So when actual click happens, jQuery calls console.debug with two parameters - Window object that was bound during click() and jQuery event that is always passed to click handler. console.debug() can accept and display multiple objects, which is exactly what you see in the developer console.
The first parameter of bind is the new context to use for this. By passing undefined you are essentially not passing the first parameter.
The second and further parameters are passed into the function as the first values.
Note also that this when in the global scope, refers to the window object.
So here, b...
console.debug.bind(undefined,this);
is identical to...
function(){ console.debug(window); }
..since you're passing this (which is window) as the first parameter to debug.
By default, when you attach an event to the element, this will automatically point to the element which caught the event, so bind shouldn't even be necessary, which is why $("a").click(a); worked without using bind.
In a codebase I see bind used to make bound copies of functions on the prototype, used as callbacks for DOM events.
Why might this idiom be used, rather than, for example, using the methods on the prototype directly?
Does this offer some benefits in terms of memory consumption/the ability to free memory use when events are unbound from DOM events?
function F() {
var onFoo = this._onFoo.bind(this); // Why?
document.getElementById('foo').onClick(onFoo);
}
F.prototype._onFoo = function () { /*...*/ }
The issue is that event handlers set their own value for this when they call the callback. That value will typically be related to the event handler, not to the object that the method is bound to. For example, in your example:
document.getElementById('foo').onClick(myObj.myFunc);
The this pointer in myFunc will be set to the DOM element that had the event handler attached (in this case, the foo element). But that isn't myObj so myFunc in that case could not access any of it's own instance variables via the this pointer (the normal way that methods access their instance data).
So, if you have a method that wants to access it's own instance data when it is called directly by an event handler, you have to do something other than just pass the method to the event handler. There are a couple ways to work around this issue.
One way of doing so it so use .bind() which returns a new stub function who's function is to set this before calling your function like this:
document.getElementById('foo').addEventListener('click', myObj.myFunc.bind(myObj));
In this case .bind() actually returns a new stub function who's function is to set the value of this to myObj before it calls myFunc.
You could also do that manually yourself like this:
document.getElementById('foo').addEventListener('click', function(e) {
myObj.myFunc();
});
But, as you can see, .bind() provides a shortcut that takes less code (which is why it was invented).
A potential disadvantage to using .bind() in some cases is that you may no longer have access to the value of this that the caller of your callback would have set itself because .bind() threw that value away and replaced it with your own. In the event handler example above, this is not an issue because the original source of the event can be accesses via the e argument that is passed to the event handler so it is not lost if you need it.
I am aware of no meaningful difference in memory consumption or garbage collection between the two methods above. Both create a new function that is used to call the original and control the value of this when calling the original function. Both will have the same garbage collection lifetime.
It appears that one thing that is confusing you is that objects in Javascript are assigned or passed by pointer (some call it by reference, but that has some connotations that don't apply here so I'll use the phrase by pointer).
var x = {};
x.myFunc = function() {console.log("hello");};
x.myFunc(); // generates "hello" in the console
var t = x.myFunc; // save reference to the function that x.myFunc currently points to
delete x.myFunc; // remove property myfunc from the x object
t(); // generates "hello" in the console
t() still works event after x.myFunc has been removed because both t and x.myFunc had a reference (or pointer) to the same function. Doing a delete x.myFunc simply removed the myFunc property from the x object. The function that x.myFunc points to will only be "freed" by the GC when there are no other references to it. But, there is another reference to that function in t, so it is not freed and t() can use it for as long as t exists.
I am working on a project that uses some JS prototyping and jQuery.
My issue, I believe, is how 'this' is being used.
In my code, I have some jQuery in a function that I will be prototyping. The jQuery looks like this:(in this code, 'target' is a jQuery object passed when IMAGE_UPLOADER is first created.)
document.getElementById(target.find('.file_selector').prop('id')).addEventListener("change", this.FileSelectHandler, false);
In this event listener, there is a function called FileSelectHandler. This function is being called just fine. However, within this function, there is a call to a second function. Here is a short version of the function:
FILE_UPLOADER.prototype.FileSelectHandler = function(e) {
this.FileDragHover(e);
}
This is where the error comes up. JS is complaining that the function 'FileDragHover' does not exist. It, of course DOES exist and is defined as follows:
FILE_UPLOADER.prototype.FileDragHover = function(e) {}
I hope this is enough info to understand the problem. If not, please let me know and I can add more.
You can use $.proxy() to pass the custom execution handler to the event handler, this inside the event handler refers to the dom element where the listener is attached - it is the same as Function.bind() but jQuery version supports IE < 9.
document.getElementById(target.find('.file_selector').prop('id')).addEventListener("change", $.proxy(this.FileSelectHandler, this), false);
The handler registration can be simplified using jQuery to
target.find('.file_selector').change($.proxy(this.FileSelectHandler, this))
If an object obj has a function func and you do obj.func() the function is called in context of obj where this in that function then referes to obj.
If you however do something like this:
var callback = obj.func;
callback();
Then callback is not called in the context of obj anymore. For browsers the context is then window. This is what is happening if you pass the function as callback to the addEventListener.
Depending how your callback is used it can be called with another context, for event listeners it is the DOM element, so this in your code refers to the DOM element.
To solve this problem you can used Function.prototype.bind (you need to check the browser support if it is usable for you or use a polyfill for it, that can be found on the mdn page), you can use jQuery.proxy or create a closure and maintain the context yourself.
The this variable refers to the invoking object not the object that the method is declared on. So if I have someObject.someFunction(); then someObject is the invoking object and the value of this in the function.
To demonstrate:
var obj1 = {
name:"obj1",
say:function(){
console.log(this.name);
}
};
var obj2 = {name:"obj2"};
obj2.say = obj1.say;
obj2.say()//logs obj2
In the above code say was declared on obj1 but invoked from obj2.
More on the this value, constructor functions and prototype here.
Take this very simple framework I am experimenting with (so that I can learn JavaScript's function prototype more in depth.)
(function(){
var app = {
ui: {
app: document.querySelector('.app'),
settings: document.querySelector('.settings'),
},
actions: 'click settings openSidebar'
,
functions: {
openSidebar: function(e){
console.log(this); // <- expected value of this is 'app.ui'
}
},
run: function(){
var a1 = this.actions.split("\n");
var a2 = this.actions.split(" ");
var self = this;
this.ui[a2[1]].addEventListener(a2[0], function(e){
app.functions.openSidebar.call(self.ui,e);
});
}
};
app.run();
})();
This works great. Output from console is:
Object {ui: Object, actions: "click settings openSidebar", functions: Object, run: function}
However, when I try to do it like this:
var self = this;
this.ui[a2[1]].addEventListener(a2[0], function(e){
app.functions.openSidebar(e);
}.bind(this));
The context of this in openSidebar() is openSidebar (aka bind has no effect). Console output:
Object {openSidebar: function}
However, when I try to use the apply function, like so:
app.functions.openSidebar.apply(self.ui,e);
It works fine (this in openSidebar is app.ui) EXCEPT that the argument (e) does not get passed, so e == undefined.
Here goes:
1. Why does not bind work (at all) in the first example?
2. Why does apply work without passing arguments (the e (event))?
And for added brownie points:
3. Why does call work as expected?
Why does not bind work (at all) in the first example?
It "works", it just doesn't do what you expect.
Inside your anon function this is indeed the value set by bind. However, when you then call a function that is also a property of an object (functions.openSidebar) then for that invocation this is automatically bound to that object inside the function (i.e. this === functions). The value of this from the parent context is never "inherited" down the call chain.
Why does apply work without passing arguments (the e (event))?
Because apply tries to pull out the arguments for the call from its own second argument e by treating it as an array. This means that its length property is checked first; your event object doesn't have a length so apply gets the undefined value produced and effectively behaves as if you had passed in an empty array. See the annotated ES5 reference for the details.
Why does call work as expected?
This is a strange question. Why would it not work? You are using it exactly like it's meant to be used.
You'll need to do this:
this.ui[a2[1]].addEventListener(a2[0], app.functions.openSidebar.bind(this));
bind returns you a new function with a manually set context of whatever you pass in. Because you're not using bind on the right function, you're calling your function with app.functions to the left of the invoked function, which henceforth is known as this inside the invoked function!
Apply takes arguments as an array, not named parameters...
I can't explain the third without saying that that is how call works!
I have a situation like this one:
talenti = $(".talenti");
filtra = $(".filtra");
wrapNavHeight = $("#wrapNav").outerHeight(true);
filtra.click(function(e) {
e.preventDefault();
$(".nasco").hide();
$("#sliding-navigation").delay(delay).show();
delay += 500;
talenti.removeClass('opened');
filtra.addClass('opened');
filtra.attr('id',"focF");
talenti.attr('id',"");
if (filtra.hasClass("opened")) {
$("#wrapNav").slideToggle("100", "linear", function(){
alert(wrapNavHeight);
$("#container").animate({"height": "+=wrapNavHeight"}, 100,function(){
$(".box").animate({"top": "+=wrapNavHeight"});
});
});
}
});
I am trying to get wrapNavHeight but alert(wrapNavHeight); outputs null; can't then assign that value to the next animate lines
Anyone?
isn't it just that you are assigning the variable the value of outerHeight at the time it's not visible? I think you need to re-evaluate outerHeight after the toggle transition. Replace
alert(wrapNavHeight);
with
alert($("#wrapNav").outerHeight(true));
see if that's any better?
Nobody else actually explained why this happens. Here's why:
It depends on:
which object is used as "this" for invocation of the function containing all the code above
which object is used as "this" for invocation of the function defined starting on line 4 of your code
In JavaScript, "global" references actually apply to the current this object (or to the "true" global object (window in web browsers) if not within a function)
Thus, if the this objects for the 2 functions I pointed out above are different, then you'll get the situation you observed.
In the browser, the default this object is usually window, but this can be changed when the function is run, such as by passing a different parameter to apply or by calling the function as a method.
Been a year since I used jQuery seriously, but if I recall right, jQuery event handlers usually rebind this to something useful related to the event (using apply-- you can do this too).
So, assuming the outer function's this hasn't been bound to anything special other than window, simply replace wrapNavHeight with window.wrapNavHeight in the inner function to achieve your desired effect.
(In practice I wouldn't actually do this, though, as a matter of style. Just declare wrapNavHeight as a var within the outer function instead, and then you'll get lexical scoping.)
Try searching.
jQuery global variable best practice & options?