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CSS animations make it possible to do creative animations of content using nothing but CSS. However, there are likely to be times when this feature isn't available, and you may wish to handle that case by using JavaScript code to simulate a similar effect. This article, based on this blog post by Chris Heilmann, demonstrates a technique for doing this.
Testing for CSS animation support
This code will check to see if CSS animation support is available:
var animation = false,
animationstring = 'animation',
keyframeprefix = '',
domPrefixes = 'Webkit Moz O ms Khtml'.split(' '),
pfx = '',
elm = document.createElement('div');
if( elm.style.animationName !== undefined ) { animation = true; }
if( animation === false ) {
for( var i = 0; i < domPrefixes.length; i++ ) {
if( elm.style[ domPrefixes[i] + 'AnimationName' ] !== undefined ) {
pfx = domPrefixes[ i ];
animationstring = pfx + 'Animation';
keyframeprefix = '-' + pfx.toLowerCase() + '-';
animation = true;
break;
}
}
}
For starters we define a few variables. We assume that animation is not supported by setting animation to false. We set the animationstring to animation which is the property we want to set later on. We create an array of browser prefixes to loop over and we set pfx to an empty string.
Then we check if the CSS animation-name property on the style collection for the element specified by the variable elm is set. This means the browser supports CSS animation without any prefix, which, to date, none of them do.
If the browser does not support non-prefixed animation and animation is still false, we iterate over all the possible prefixes, since all the major browsers are currently prefixing this property and changing its name to AnimationName instead.
Once this code is finished running, the value of animation will be false if CSS animation support isn't available, or it will be true. If it is true then both the animation property name and the keyframe prefix will be the right ones. So if you use a new Firefox, the property will be MozAnimation and the keyframe prefix -moz- and with Chrome it'll be WebkitAnimation and -webkit-. Notice browsers don't make it easy with the switching between camelCase and hyphen-ation.
Animating using the correct syntax for different browsers
Now that you know if CSS animation is supported or not, we can animate.
if( animation === false ) {
// animate in JavaScript fallback
} else {
elm.style[ animationstring ] = 'rotate 1s linear infinite';
var keyframes = '@' + keyframeprefix + 'keyframes rotate { '+
'from {' + keyframeprefix + 'transform:rotate( 0deg ) }'+
'to {' + keyframeprefix + 'transform:rotate( 360deg ) }'+
'}';
if( document.styleSheets && document.styleSheets.length ) {
document.styleSheets[0].insertRule( keyframes, 0 );
} else {
var s = document.createElement( 'style' );
s.innerHTML = keyframes;
document.getElementsByTagName( 'head' )[ 0 ].appendChild( s );
}
}
This code looks at the value of animation; if it's false, we know we need to use our JavaScript fallback code to perform our animation. Otherwise, we can use JavaScript to create the desired CSS animation effects.
Setting the animation property is easy; simply update its value in the style collection. However, adding keyframes is trickier, since they're not defined using traditional CSS syntax (which makes them more flexible, but harder to define from script).
To define our keyframes using JavaScript, we need to write them out as a CSS string. All we do is set up a keyframes variable, prefixing each attribute as it's constructed. This variable, once constructed, contains the complete description of all the keyframes needed by our animation sequence.
The next task is to actually add the keyframes to the page's CSS. The first thing to do is look to see if there's already a style sheet on the document; if so, we simply insert the keyframe description into that stylesheet; this is done in lines 13-15.
If there isn't already a style sheet, a new <style> element is created, and its content is set to the value of keyframes. Then the new <style> element is inserted into the document's <head>, thereby adding the new style sheet to the document.