type TextProps = { editable: false }
               | { editable: true, onEdit: (newText: string) => void }

class TextComponent extends React.Component<TextProps, {}> {
    render() {
        return <span>Some Text..</span>;
    }
}

ReactDOM.render(
    <TextComponent editable={true} />, // editable is of type boolean not of type "true"
    document.getElementById("example")
);