CanUseDom.jsx

import React from "react";
import ExecutionEnvironment from "react/lib/ExecutionEnvironment";

React.createClass({
    componentWillMount() {
        if (ExecutionEnvironment.canUseDOM) {
            // ... has dom .. 
        } else {
            //Server side rendering
        }
    },
    render() {
        return <SomeComponent />;
    }
});