jQuery is basically a wrapper that returns an object with many methods. Most of them are not that straightforward, if you want to understand more deeply, you don't have many choices except using the console and going through the source code which you can find here: https://code.jquery.com/jquery-1.12.4.js. Ideally, use uncompressed version. For some methods, it can be quite long to get to the bottom of it. The way the Click callback works is hidden pretty deep.
You can find it this way:
In the console, enter $("p").click. You'll get:
ƒ ( data, fn ) {
return arguments.length > 0 ?
this.on( name, null, data, fn ) :
this.trigger( name );
}
Which comes from here in the source code:
jQuery.each( ( "blur focus focusin focusout load resize scroll unload click dblclick " +
"mousedown mouseup mousemove mouseover mouseout mouseenter mouseleave " +
"change select submit keydown keypress keyup error contextmenu" ).split( " " ),
function( i, name ) {
// Handle event binding
jQuery.fn[ name ] = function( data, fn ) {
return arguments.length > 0 ?
this.on( name, null, data, fn ) :
this.trigger( name );
};
} );
So because you have at least an argument, it runs: this.on( name, null, data, fn ), where this is your jQuery object $('p'), name is 'click', and data is your function(event). So onto this.on():
console.log($('p').on);
ƒ ( types, selector, data, fn ) {
return on( this, types, selector, data, fn );
}
Here, function on isn't global, so it's in jQuery's closure, so back to the source code where you can find:
function on( elem, types, selector, data, fn, one ) {
...
where elem is your jQuery object $('p'), types is 'click', selector is null, data is your function(e) and fn is null. This leads to:
elem.each( function() {
jQuery.event.add( this, types, fn, data, selector );
} );
So you can find:
jQuery.event = {
global: {},
add: function( elem, types, handler, data, selector ) {
...
Where you can find an addEventListener :
elem.addEventListener( type, eventHandle, false );
On addEventListener, the callback has the event parameter, which is native javascript. In jQuery, the callback is eventHandle, so let's find this one:
eventHandle = elemData.handle = function( e ) {
// Discard the second event of a jQuery.event.trigger() and
// when an event is called after a page has unloaded
return typeof jQuery !== "undefined" &&
( !e || jQuery.event.triggered !== e.type ) ?
jQuery.event.dispatch.apply( eventHandle.elem, arguments ) :
undefined;
};
So it returns the function dispatch, so now the callback is this :
jQuery.event.dispatch.apply( eventHandle.elem, arguments ) , where arguments is e (the original addEventListener Event). So find dispatch:
dispatch: function( event ) {
// Make a writable jQuery.Event from the native event object
event = jQuery.event.fix( event );
...
So what is this event.fix:
fix: function( event ) {
if ( event[ jQuery.expando ] ) {
return event;
}
// Create a writable copy of the event object and normalize some properties
var i, prop, copy,
type = event.type,
originalEvent = event,
fixHook = this.fixHooks[ type ];
In here you find
event = new jQuery.Event( originalEvent );
jQuery.Event = function( src, props ) {
...
Where the event that is passed as a parameter of click is defined. You can test it by adding properties on jQuery.Event.prototype. Like this for example:
jQuery.Event.prototype.prop = 'newProp';
So, to sum up, the event in function(event), is an instance of jQuery.Event.
See
console.log($('p').click);
console.log($('p').on);
console.log(jQuery.Event)
jQuery.Event.prototype.prop = 'test';
$('p').click(function(event){console.log(event.prop)});
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<p>test</p>
For Ajax, it's probably a bit more straightforward, but again, if you want to know exactly, not much else you can do but go through the source code.
