foo.jsx

/** @jsx: React.DOM */
var React = require('react');

var IfThingy = React.createClass({
  render: function() {
    var ifThingy;
    if (this.props.someVar) {
      ifThingy = <h1>Some Var Is {this.props.someVar}</h1>;
    }
    else {
      ifThingy = <h1>Some var is falsy :(</h1>;
    }
    return <div>
    {ifThingy}
    </div>
  }
})

Since you represent the DOM with js objects, you just use javascript for your control flow. You don't do any icky matching brackets or anything. It's Just JavaScript!