How do you run javascript with only a CSS class?

Tags: javascript,html,css,json

Problem :

I am working on a web site project which requires that the web page display tooltips on hover. For reasons I won't go into here, I decided to use a library from a site called The library works great but I need to make one small change to the way it works, which will make it a perfect fit for my application. Trouble is, I can't figure out how it works!

Everything I've read says that you can't execute javascript code from within CSS. But that seems to be exactly what this library does. To create a tooltip for any element (anchor, div, span, etc), all this library requires you to do is:

  1. Include a <script src= > tag to the library file
  2. Add two class names to the HTML element that will host the tooltip, one called "showTip" and the other is a key into a JSON object containing the tooltip text
  3. Add the tooltip text to the JSON object mentioned above
  4. Create a style to format the tooltip how you want it to look

If you'll notice, nowhere in these steps is mention of any event handlers. Moreover, there's no class (that I can find) called showTip. There's no JQuery or other dependencies. So how does the javascript get executed?

I don't want to jump in and start changing the library willy-nilly without knowing how it works, and I've been pulling out my hair trying to figure it out. Can one of you smarter-than-me folks explain it?

Solution :

From the steps you listed, it seems as if Dynamic Web Coding library uses the JavaScript to apply CSS attributes into elements. You can not run JavaScript inside CSS, but you can certainly apply CSS using JavaScript!

What the library probably does is look at all of the elements with class showTip with JavaScript using document.getElementsByClassName("showTip"). Then, it looks at the other class on that element and assumes that as the key to the tooltip with the tooltip text.

Then, it probably creates an element for the tooltip using document.createElement() and then injects that into the document using document.appendChild() or document.insertBefore(). They can probably add some class to the tooltip so your CSS rules are applied using the .className property.

It also likely used document.addEventListener() to listen for when the user hovers over and leaves the element. You don't need to add this code because the <script> tag you added has called document.addEventListener() for you.

All of these things require something known as the DOM which is something JavaScript coders use to manipulate the HTML document. This is actually really powerful and cool, so you can check out a good tutorial on it at TutorialsPoint.

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