How to change ReactJS styles dynamically?
Ok, finally found the solution.
Probably due to lack of experience with ReactJS and web development...
var Task = React.createClass({
render: function() {
var percentage = this.props.children + '%';
....
<div className="ui-progressbar-value ui-widget-header ui-corner-left" style={{width : percentage}}/>
...
I created the percentage variable outside in the render function.