Setting the transition duration

HTML

<div id="transitionee"></div>

CSS

#transitionee { 
    background: red; 
    width: 200px;
    height: 1em;
    -webkit-transition-timing-function: ease-out;    
} 
#transitionee:hover {
    width: 400px;
}

JavaScript

document.getElementById('transitionee').style.transitionDuration = '1s';