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Changing CSS using JavaScript

by kirupa | 31 March 2011

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There will be times when you want to change how your application look when users are interacting with it. Sure, you can use CSS pseudo classes, but what if you want to trigger a visual change on something other than the handful of cases where pseudo classes would apply?

Fortunately, to address what pseudo classes cannot do, you can use JavaScript to apply CSS rules to elements to change their appearance in a whole lot more conditions. For example, let's say I have a simple page with a button displayed on a gray background:

When the button is clicked, the background is changed from gray to some magenta/pink color:

The way this change was done was not by setting some inline style using JavaScript. Instead, while JavaScript is still used, it is used to swap out one CSS style rule with another:

.backgroundOne {
}
.backgroundTwo {
}

The backgroundOne rule corresponds to the gray background, and the backgroundTwo rule corresponds to the magenta background. Determining which to apply and actually applying it was handled entirely by JavaScript! In this tutorial, you will learn the secrets passed down from my forefathers on how to do that.

Getting Started
If you want to follow-along, go ahead and create a new HTML document and copy/paste the following code into it:

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

<head>
<meta charset="utf-8">
<title>Changing CSS Using JavaScript</title>
<style type="text/css">
#mainDiv {
}
#mainDiv p {
}
#mainDiv button {
}
.backgroundOne {
}
.backgroundTwo {
}

</style>
</head>

<body>
<div id="mainDiv" class="backgroundOne">
<p>I am so bored...</p>
<button id="clickButton">click me</button>
</div>

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

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

function doSomething() {

}

</script>
</body>

</html>

When you preview this HTML in your browser, you will basically see something that looks as follows:

I am not going to go over the HTML or the CSS that exists in great detail, for it should be straightforward. What I will want you to focus on is the JavaScript:

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

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

function doSomething() {
}

</script>

The myButton and myDiv variables, respectively, get a reference to the button and div elements that live in our HTML. You can learn more on that by reading the Referencing HTML Elements using JavaScript tutorial.

The other important code deals with reacting to our button click. When our button is clicked, the doSomething function gets called. It is inside this function where everything we do will live.

Changing CSS using JavaScript
The easiest and most direct way to change your CSS via JavaScript is to set the className property on an element to the class selector whose CSS rule you wish to apply. In our case, that would be backgroundTwo:

myDiv.className = "backgroundTwo";

Your full script would look as follows:

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

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

function doSomething() {
}

</script>

When you preview your application, you will see that your div's background changes to the color specified by the backgroundTwo rule when your button is clicked. Wohoo!

While the approach described here works most of the time, there is one side-effect. If your element already has some CSS rules based on a class selector affecting it, this approach obliterates those with the new class selector you specify. Talk about disrespecting the local culture!

To avoid that, use the following approach where you add your new class selector to the element without getting rid of any class selectors that may already exist:

myDiv.className += "backgroundTwo";

The main change is that the destructive = has been changed to the additive +=. To remove a CSS selector that has been applied to your element, you have two ways of handling this as well. One way is by just setting the className to an empty string:

myDiv.className = '';

This approach is OK if you want to clear all of your selectors, but if you want to only remove a particular selector, the second approach may work better. To only remove the class selector you are interested in while keeping the other selectors intact, use the following approach instead:

myDiv.className = myDiv.className.replace("backgroundTwo",'');

What you are doing is calling the replace function on className to find the string representing the class selector you wish to replace (backgroundTwo) and replace it with a blank space (' '). In fact, this approach works even in cases where you may only have one class selector specified in your className property. This code will find that one property and replace it with an empty string.

Avoid Setting CSS Values in JavaScript
I've been very careful about describing how to work with CSS only if it exists in the markup. One thing I encourage you to avoid is setting CSS values and properties directly using JavaScript. The following is an example of what not to do where I set the background on our div to the color directly without using a CSS rule:

myDiv.style.backgroundColor = "#990033";

To set the background color, I used the backgroundColor property. Equivalent properties exist for all of the other CSS properties you may want to work with.

The reason I discourage this approach is because it makes your code less maintainable. If you ever decide to change the color of the background when the button is clicked, you will need to ensure that any locations in code are updated as well. If you just set the appropriate selector name instead, you can be assured that updating the value in your CSS rule will ensure that any code that references it will get the updated value from your CSS without you having to manually do anything extra.

Conclusion
Well, that's all there is to mixing JavaScript and CSS as part of a nutritious application. I presented several ways of both setting a selector as well as removing it, so be sure to use the one that is most appropriate. If you are unsure which approach to take, just use the more general approach for adding a selector:

myDiv.className += "backgroundTwo";

Also, use the more general approach for removing a selector:

myDiv.className = myDiv.className.replace("backgroundTwo",'');

Just like buying a black colored Ford Model T, you can't go wrong with these two approaches.

You can see a live version of the example I took screenshots from. My example has a slight variation to the code you saw presented here because I wanted to enable toggling the background each time you clicked on the button.

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