Persistence of JQuery Functions

Question

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 statemen

Answer 1

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        : 
, 
    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, 

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

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 ->

, 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 


        this.animate(); // fails as 
 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.