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Getting Your Feet Wet in HTML 5 - Page 3

by kirupa | 23 March 2011

Have questions? Discuss this HTML 5 tutorial with others on the forums.

In the previous page, you made your content look nice by using some CSS magic. In this page, let's focus on the interactive side of our application and look at pseudo-classes and JavaScript!

Using Pseudo-Classes
The last little bit of CSS tomfoolery we will talk about are pseudo-classes. When you hovered over the click me button or pressed your mouse on it to click, you may have noticed that the button's look changed:

These variations in the look are defined in CSS, and through what are known as pseudo-classes, the appropriate CSS is applied by your browser when you are interacting with the button. The two pseudo classes that are guilty for our button example are hover and active:

<style type="text/css">
#mainContent {
}
.buttonStyle {
}

.buttonStyle:hover {
}
.buttonStyle:active {
}

</style>

Go ahead and add the buttonStyle:hover and buttonStyle:active blocks to your CSS. The look of the button when you hover over it is defined in buttonStyle:hover, and the look of the button when you are pressing it is defined in buttonStyle:active. One thing to note is that both the hover and active cases are simply modifications on top of the base buttonStyle class.

The buttonStyle class has 8 properties defined. Both the hover and active pseudo-classes modify 4 of those 8 properties by giving it their own value. In a case where the child modifies the values of properties defined by the parent, the child always wins. The remaining 4 properties the pseudo-classes do not implement will simply be inherited from buttonStyle itself. This is another example of the cascading nature of style sheets at work!

After adding these lines, you can preview your application and notice how the mouse cursor and the button's look changes when you hover over or click it.

Adding Interactivity using JavaScript
We are almost done. The last thing we are going to do is figure out how to display the hello, world! text when the button is clicked. For this, you will need to use some JavaScript. First, just like we did for styles, let's add the script region where our JavaScript will live.

Below your closing div tag and above your closing body tag, add your script block:

<body>

<div id="mainContent">
<p>?</p>
<button class="buttonStyle">click me</button>
</div>
<script>


</script>
</body>

Within this area, the scripts that you are going to use will live. Like you saw mentioned earlier, what we want to do is display hello, world! when somebody clicks on our button. Let's divide that want into two buckets of tasks:

Doing something when the button is clicked.
Changing the text in our p tag when the button is clicked.

Let's look at those two tasks in greater detail.

Reacting to a Button Click
In order to react to some particular action, you'll need to enter the world of events and event handlers. The event is us clicking the button. The reaction to that click is handled by the event handler. First, give your button an id value. Let's call it clickButton;

<button id="clickButton" class="buttonStyle">click me</button>

Once you have given your button an id value, you can reference it easily via code. Inside your script block, add the following line of JavaScript:

<script>
var myButton = document.getElementById("clickButton");
</script>

What you are doing is using JavaScript's getElementById function to get a reference to your button which is declared in markup and storing that reference in a variable called myButton. Once you have this reference to your button, you can listen for clicks on it and handle it appropriately.

To listen for a click event and to react to it, add the following lines of code directly below your myButton declaration:

<script>

var myButton = document.getElementById("clickButton");
myButton.addEventListener('click', doSomething, false)

function doSomething() {
}

</script>

What you are doing is listening to the click event on myButton by using addEventListener. The addEventListener function takes the event, the event handler name, and a boolean true/false specifying whether you want to capture this event or not.

Our event handler is called doSomething, and you can see that same function name specified in our addEventListener call. This function will get called everything time the event you are listening for gets fired on the element you are listening on. In English, doSomething gets called every time you click on your button.

If you preview your application now and click on the button, you'll see a friendly dialog that says hello, world!:

This isn't quite what we want, but at least this can act as a good sanity check that your event and event handler association are working. Ok, let's make the final change where we change the text in our p tag.

Changing the InnerText of an Element
First, just like what we did for the button, we need to get a reference to our p tag in our script. This means, giving our p tag an id value and then calling getElementyById on it.

Let's give our p tag the id helloText:

<p id="helloText">?</p>

Once you have made that change in your HTML, declare the myText variable and change the code in your doSomething function to change our p tag's innerText instead of displaying an alert:

<script>

var myButton = document.getElementById("clickButton");
var myText = document.getElementById("helloText");

myButton.addEventListener('click', doSomething, false)

function doSomething() {
}


</script>

As you can see, the myText variable will store a reference to the element whose id is helloText, and as we both know, that is our p tag. Once we have a reference to our p tag, the way you change its value (currently a "?") is by setting a new value into its textContent property:

myText.textContent = "hello, world!";

As you can see, the new value we are going to set is "hello, world!". When you run your application this time around and click on your button, you will see your p tag now display what you set out to display many pages earlier:

Conclusion
Wow, we have covered a lot of ground in this tutorial. The goal of this tutorial was just to give you a quick tour of what is easily possible using a little bit of HTML, CSS, and JavaScript. In subsequent tutorials, we'll spend more time sightseeing and stopping at the various points of interest as opposed to rushing through them, so stay tuned for more guided tours.

In case you are curious or stuck, here is the final code with everything:

<!DOCTYPE html>
<html lang="en-us">

<head>
<meta charset="utf-8">
<title>Hello...</title>
<style type="text/css">
#mainContent {
}
.buttonStyle {
}

.buttonStyle:hover {
}
.buttonStyle:active {
}

</style>
</head>

<body>
<div id="mainContent">
<p id="helloText">?</p>
<button id="clickButton" class="buttonStyle">click me</button>
</div>
<script>

var myButton = document.getElementById("clickButton");
var myText = document.getElementById("helloText");

myButton.addEventListener('click', doSomething, false)

function doSomething() {
}


</script>

</body>
</html>

Just a final word before we wrap up. If you have a question and/or want to be part of a friendly, collaborative community of over 220k other developers like yourself, post on the forums for a quick response!


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