react-router - pass props to handler component

UPDATE

Since new release, it's possible to pass props directly via the Route component, without using a Wrapper. For example, by using render prop.

Component:

class Greeting extends React.Component {
  render() {
    const {text, match: {params}} = this.props;

    const {name} = params;

    return (
      <React.Fragment>
        <h1>Greeting page</h1>
        <p>
          {text} {name}
        </p>
      </React.Fragment>
    );
  }
}

Usage:

<Route path="/greeting/:name" render={(props) => <Greeting text="Hello, " {...props} />} />

Codesandbox Example

---

OLD VERSION

My preferred way is wrap the Comments component and pass the wrapper as a route handler.

This is your example with changes applied:

var Dashboard = require('./Dashboard');
var Comments = require('./Comments');

var CommentsWrapper = React.createClass({
  render: function () {
    return (
      <Comments myprop="myvalue"/>
    );
  }
});

var Index = React.createClass({
  render: function () {
    return (
      <div>
        <header>Some header</header>
        <RouteHandler/>
      </div>
    );
  }
});

var routes = (
  <Route path="/" handler={Index}>
    <Route path="comments" handler={CommentsWrapper}/>
    <DefaultRoute handler={Dashboard}/>
  </Route>
);

ReactRouter.run(routes, function (Handler) {
  React.render(<Handler/>, document.body);
});

This is the solution from Rajesh, without the inconvenient commented by yuji, and updated for React Router 4.

The code would be like this:

<Route path="comments" render={(props) => <Comments myProp="value" {...props}/>}/>

Note that I use render instead of component. The reason is to avoid undesired remounting. I also pass the props to that method, and I use the same props on the Comments component with the object spread operator (ES7 proposal).

If you'd rather not write wrappers, I guess you could do this:

class Index extends React.Component { 

  constructor(props) {
    super(props);
  }
  render() {
    return (
      <h1>
        Index - {this.props.route.foo}
      </h1>
    );
  }
}

var routes = (
  <Route path="/" foo="bar" component={Index}/>
);

Copying from the comments by ciantic in the accepted response:

<Route path="comments" component={() => (<Comments myProp="value" />)}/>

This is the most graceful solution in my opinion. It works. Helped me.