In Javascript/jQuery what does (e) mean?

I am new to JavaScript/jQuery and I've been learning how to make functions. A lot of functions have cropped up with (e) in brackets. Let me show you what I mean:

$(this).click(function(e) {
    // does something
});

It always appears that the function doesn't even use the value of (e), so why is it there so often?

e is the short var reference for event object which will be passed to event handlers.

The event object essentially has lot of interesting methods and properties that can be used in the event handlers.

In the example you have posted is a click handler which is a MouseEvent

$(<element selector>).click(function(e) {
    // does something
    alert(e.type); //will return you click
}

DEMO - Mouse Events DEMO uses e.which and e.type

Some useful references:

http://api.jquery.com/category/events/

http://www.quirksmode.org/js/events_properties.html

http://www.javascriptkit.com/jsref/event.shtml

http://www.quirksmode.org/dom/events/index.html

http://www.w3.org/TR/DOM-Level-3-Events/#event-types-list

DISCLAIMER: This is a very late response to this particular post but as I've been reading through various responses to this question, it struck me that most of the answers use terminology that can only be understood by experienced coders. This answer is an attempt to address the original question with a novice audience in mind.

Intro

The little '(e)' thing is actually part of broader scope of something in Javascript called an event handling function. Every event handling function receives an event object. For the purpose of this discussion, think of an object as a "thing" that holds a bunch of properties (variables) and methods (functions), much like objects in other languages. The handle, the 'e' inside the little (e) thing, is like a variable that allows you to interact with the object (and I use the term variable VERY loosely).

Consider the following jQuery examples:

$("#someLink").on("click", function(e){ // My preferred method
    e.preventDefault();
});

$("#someLink").click(function(e){ // Some use this method too
    e.preventDefault();
});

Explanation

• "#someLink" is your element selector (which HTML tag will trigger this).
• "click" is an event (when the selected element is clicked).
• "function(e)" is the event handling function (on event, object is created).
• "e" is the object handler (object is made accessible).
• "preventDefault()" is a method (function) provided by the object.

What's happening?
When a user clicks on the element with the id "#someLink" (probably an anchor tag), call an anonymous function, "function(e)", and assign the resulting object to a handler, "e". Now take that handler and call one of its methods, "e.preventDefault()", which should prevent the browser from performing the default action for that element.

Note: The handle can pretty much be named anything you want (i.e. 'function(billybob)'). The 'e' stands for 'event', which seems to be pretty standard for this type of function.

Although 'e.preventDefault()' is probably the most common use of the event handler, the object itself contains many properties and methods that can be accessed via the event handler.

Some really good information on this topic can be found at jQuery's learning site, http://learn.jquery.com. Pay special attention to the Using jQuery Core and Events sections.

e doesn't have any special meaning. It's just a convention to use e as function parameter name when the parameter is event.

It can be

$(this).click(function(loremipsumdolorsitamet) {
    // does something
}

as well.