Detect CSS transitions using Javascript (and without modernizr)?

Perhaps something like this. Basically it's just looking to see if the CSS transition property has been defined:

function supportsTransitions() {
    var b = document.body || document.documentElement,
        s = b.style,
        p = 'transition';

    if (typeof s[p] == 'string') { return true; }

    // Tests for vendor specific prop
    var v = ['Moz', 'webkit', 'Webkit', 'Khtml', 'O', 'ms'];
    p = p.charAt(0).toUpperCase() + p.substr(1);

    for (var i=0; i<v.length; i++) {
        if (typeof s[v[i] + p] == 'string') { return true; }
    }

    return false;
}

Adapted from this gist. All credit goes there.

3 ways of doing so:

var supportsTransitions  = (function() {
    var s = document.createElement('p').style, // 's' for style. better to create an element if body yet to exist
        v = ['ms','O','Moz','Webkit']; // 'v' for vendor

    if( s['transition'] == '' ) return true; // check first for prefeixed-free support
    while( v.length ) // now go over the list of vendor prefixes and check support until one is found
        if( v.pop() + 'Transition' in s )
            return true;
    return false;
})();

console.log(supportsTransitions) // 'true' on modern browsers

OR:

var s = document.createElement('p').style,
        supportsTransitions = 'transition' in s ||
                              'WebkitTransition' in s ||
                              'MozTransition' in s ||
                              'msTransition' in s ||
                              'OTransition' in s;

console.log(supportsTransitions);  // 'true' on modren browsers

If you actually want to use the right prefixed, use this:

function getPrefixed(prop){
    var i, s = document.createElement('p').style, v = ['ms','O','Moz','Webkit'];
    if( s[prop] == '' ) return prop;
    prop = prop.charAt(0).toUpperCase() + prop.slice(1);
    for( i = v.length; i--; )
        if( s[v[i] + prop] == '' )
            return (v[i] + prop);
}

// get the correct vendor prefixed property
transition = getPrefixed('transition');
// usage example
elment.style[transition] = '1s';

As of 2015, this one-liner should do the deal (IE 10+, Chrome 1+, Safari 3.2+, FF 4+ and Opera 12+):-

var transitionsSupported = ('transition' in document.documentElement.style) || ('WebkitTransition' in document.documentElement.style);