Better React ClassSet

gistfile1.js

// Lets you call classSet with strings *and* objects
// Usage: classSet({a: true, b: false}, 'class1', null, 'class2');  class="a class1 class2"

function classSet() {
    var classes = [],
        arg, x, key;

    for( x = 0; x < arguments.length; x++ ) {
        arg = arguments[ x ];
        if( typeof arg === 'object' ) {
            for( key in arg ) {
                if( arg[ key ] ) {
                    classes.push( key );
                }
            }
        } else if( arg ) {
            classes.push( arg );
        }
    }

    return classes.join(' ');
}