Why do you have to pass the event object as a parameter?

Question

I'm learning how to manipulate events in JavaScript and I'm wondering "why do you have to pass the event object as a parameter (argument) into a function when using event handling?"

Here's an example of what I am talking about:

<script type="text/javascript">
    document.getElementById('button_1').onclick = (function (event) {
        alert("The event is: " + "on" + event.type);
    });
</script>

I wrote the code above and I pretty much understand what it does. I just don't understand the whole (event) passing. I thought of this as a way of assigning an anonymous function to the button_1.onclick event handler. Does the event handler try to pass in an event before it gets assigned or?... I'm having a difficult time understanding this. If someone could please clarify this for me I would be grateful.

[I tried searching it on Google but found very complex explanations and examples. Only a simple-to-intermediate explanation would help.] =)

Thank you very much.

Answer 1

The Ever-Present Event, Whether You Like it or Not

The event is always present, even when you don't provide a name:

$(".foo").on("click", function(){
  alert( arguments[0].type );
});

That is the same as saying this:

$(".foo").on("click", function(event){
  alert( event.type );
});

The event object is already being passed to your callback (whether your provide a name for it or not), you can choose to not use it if you like. For instance, if we looked to a jQuery onClick method:

$(".foo").on("click", function(){
  /* Do stuff */
});

Making Use of It

You'll note that I have no event object referenced in my callback. I'm not required to. However, if I want to use it, for whatever purpose, I should give it a name:

$(".foo").on("click", function(myEvent){
  myEvent.preventDefault();
  myEvent.stopPropagation();
});

Now that I have granted myself access to the event details, I can prevent the default behavior that would result from the event, and I can also stop the event from bubbling up the DOM to other elements.

Practical Example

Suppose we wanted to listen for click events on an element:

$("#bigSquare").on("click", function(event){
  /* Do something */
});

Click events happen on an element when you click the element itself, or any of its children. Now suppose this element had two children:

<div id="bigSquare">
  <div id="redSquare"></div>
  <div id="blueSquare"></div>
</div>

Clicking any of these, the big square, the red square, or the blue square will cause the "click" event on the big square - after it causes the click event on whichever element you clicked first (events bubble up the DOM).

We could determine which element was the target in any click event via the event itself:

$("#bigSquare").on("click", function(event){
  alert( event.target.id );
});

Note here how we're accessing the ID of the target that raised the event. If you click on the red square, when that event bubbles up to the big square, we will see alerted "redSquare". The same goes for the blue square. If you click that, the event will bubble up to the big square and we will see alerted "blueSquare".

You can test this online via the following demo: http://jsbin.com/ejekim/edit#javascript,live

Try clicking the orange, red, or blue square to see what is alerted.

Answer 2

You are not passing the event parameter anywhere. You are just making a function that takes one parameter, called event.

When the browser calls the event handlers, it calls the function(s) assigned to it, and passes the event object to it as the 1st parameter.

P.S. You don't need the () around your function.

document.getElementById('button_1').onclick = function (event) {
    alert("The event is: " + "on" + event.type);
};