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)
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.
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
.
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.