Simple

See https://github.com/reasonml-community/reason-react-example for examples you can clone and run. This page copy pastes a few from there for ease of reading.

let component = ReasonReact.statelessComponent "Page";
let handleClick _event _self => Js.log "clicked!";
let make ::message _children => {
  ...component,
  render: fun self =>
    <div onClick=(self.handle handleClick)> (ReasonReact.stringToElement message) </div>
};

Usage in another file:

ReactDOMRe.renderToElementWithId <Page message="Hello!" /> "index";

In the same file, you'd do:

ReactDOMRe.renderToElementWithId (make message::"Hello!" [||]) "index";