[javascript] 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?

It's a reference to the current event object

Today I just wrote a post about "Why do we use the letters like “e” in e.preventDefault()?" and I think my answer will make some sense...

At first,let us see the syntax of addEventListener

Normally it will be: target.addEventListener(type, listener[, useCapture]);

And the definition of the parameters of addEventlistener are:

type :A string representing the event type to listen out for.

listener :The object which receives a notification (an object that implements the Event interface) when an event of the specified type occurs. This must be an object implementing the EventListener interface, or a JavaScript function.

(From MDN)

But I think there is one thing should be remarked: When you use Javascript function as the listener, the object that implements the Event interface(object event) will be automatically assigned to the "first parameter" of the function.So,if you use function(e) ,the object will be assigned to "e" because "e" is the only parameter of the function(definitly the first one !),then you can use e.preventDefault to prevent something....

let us try the example as below:

<p>Please click on the checkbox control.</p>
<form>
    <label for="id-checkbox">Checkbox</label>
    <input type="checkbox" id="id-checkbox"/>

    </div>
</form>
<script>
    document.querySelector("#id-checkbox").addEventListener("click", function(e,v){
         //var e=3;
         var v=5;
         var t=e+v;
         console.log(t);
        e.preventDefault();

    }, false);
</script>

the result will be : [object MouseEvent]5 and you will prevent the click event.

but if you remove the comment sign like :

<script>
       document.querySelector("#id-checkbox").addEventListener("click", function(e,v){
            var e=3;
            var v=5;
            var t=e+v;
            console.log(t);
            e.preventDefault();

       }, false);
   </script>

you will get : 8 and an error:"Uncaught TypeError: e.preventDefault is not a function at HTMLInputElement. (VM409:69)".

Certainly,the click event will not be prevented this time.Because the "e" was defined again in the function.

However,if you change the code to:

<script>
       document.querySelector("#id-checkbox").addEventListener("click", function(e,v){
            var e=3;
            var v=5;
            var t=e+v;
            console.log(t);
            event.preventDefault();

       }, false);
   </script>

every thing will work propertly again...you will get 8 and the click event be prevented...

Therefore, "e" is just a parameter of your function and you need an "e" in you function() to receive the "event object" then perform e.preventDefault(). This is also the reason why you can change the "e" to any words that is not reserved by js.

In that example, e is just a parameter for that function, but it's the event object that gets passed in through it.

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

In reference to the above code
$(this) is the element which as some variable.
click is the event that needs to be performed.
the parameter e is automatically passed from js to your function which holds the value of $(this) value and can be used further in your code to do some operation.

In jQuery e short for event, the current event object. It's usually passed as a parameter for the event function to be fired.

Demo: jQuery Events

In the demo I used e

$("img").on("click dblclick mouseover mouseout",function(e){
        $("h1").html("Event: " + e.type);
}); 

I may as well have used event

 $("img").on("click dblclick mouseover mouseout",function(event){
            $("h1").html("Event: " + event.type);
    }); 

Same thing!

Programmers are lazy we use a lot of shorthand, partly it decreases our work, partly is helps with readability. Understanding that will help you understand the mentality of writing code.

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.

The e argument is short for the event object. For example, you might want to create code for anchors that cancels the default action. To do this you would write something like:

$('a').click(function(e) {
    e.preventDefault();
}

This means when an <a> tag is clicked, prevent the default action of the click event.

While you may see it often, it's not something you have to use within the function even though you have specified it as an argument.