How to do fade-in and fade-out with JavaScript and CSS

Tags: javascript,css,xhtml

Problem :

I want to make an HTML div tag fade in and fade out.

I have some code that fades out, but when I fade in, the opacity of the div stays at 0.1 and doesn't increase.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"

 <html xmlns="" lang="en">
    <title>Fade to Black</title>
    <script type="text/javascript">
        function slidePanel(elementToSlide, slideSource)
            var element = document.getElementById(elementToSlide);

            if(element.up == null || element.up == false) {
                setTimeout("fadeOut(\"" + elementToSlide + "\")", 100);
                element.up = true;
                slideSource.innerHTML = "Bring it down";
            } else {
                setTimeout("fadeIn(\"" + elementToSlide + "\")", 100);
                element.up = false;
                slideSource.innerHTML = "Take it up";

        function fadeIn(elementToFade)
            var element = document.getElementById(elementToFade);

   += 0.1;
            if( > 1.0) {
       = 1.0;
            } else {
                setTimeout("fadeIn(\"" + elementToFade + "\")", 100);

        function fadeOut(elementToFade)
            var element = document.getElementById(elementToFade);

   -= 0.1;
            if( < 0.0) {
       = 0.0;
            } else {
                setTimeout("fadeOut(\"" + elementToFade + "\")", 100);
        <div id="slideSource"
              style="width:150px; height:20px;
                    text-align:center; background:green"
             onclick="slidePanel('panel', this)">
            Take It up
        <div id="panel"
              style="width:150px; height:130px;
                    text-align:center; background:red;

What am I doing wrong and what is the best way to fade in and fade out an element?

Solution :

Ok, I've worked it out = parseFloat( + 0.1;

Should be used instead of += 0.1;

Same with = parseFloat( - 0.1;

Instead of -= 0.1;

Because opacity value is stored as string, not as float. I'm still not sure though why the addition has worked.

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