btrc-hub/frontend/src/components/WikiBody.tsx

import ReactMarkdown from 'react-markdown'
import { Link } from 'react-router-dom'

type Props = { title: string
	       body?: string }


export default ({ title, body }: Props) => (
    <ReactMarkdown components={{ a: (
	({ href, children }) => (['/', '.'].some (e => href?.startsWith (e))
				 ? <Link to={href!}>{children}</Link>
				 : <a href={href} target="_blank" rel="noopener noreferrer">{children}</a>)) }}>
      {body || `このページは存在しません。[新規作成してください](/wiki/new?title=${ encodeURIComponent (title) })。`}
    </ReactMarkdown>)