What are stateless components?

Stateless components (a flavor of “reusable” components) are nothing more than pure functions that render DOM based solely on the properties provided to them.

const StatelessCmp = props => {
  return (
    <div className="my-stateless-component">
    {props.name}: {props.birthday}
    </div>
  );
};

// ---
ReactDOM.render(
  <StatelessCmp name="Art" birthday="10/01/1980" />,
  document.getElementById('main')
);

This component has no need for any internal state — let alone a constructor or lifecycle handlers. The output of the component is purely a function of the properties provided to it.