gem-hunter

React component to render markdown.

• safe by default (no dangerouslySetInnerHTML or XSS attacks)
• components (pass your own component to use instead of <h2> for ## hi)
• plugins (many plugins you can pick and choose from)
• compliant (100% to CommonMark, 100% to GFM with a plugin)

• What is this?
• When should I use this?
• Install
• Use
• API
  • Markdown
  • MarkdownAsync
  • MarkdownHooks
  • defaultUrlTransform(url)
  • AllowElement
  • Components
  • ExtraProps
  • HooksOptions
  • Options
  • UrlTransform
• Examples
  • Use a plugin
  • Use a plugin with options
  • Use custom components (syntax highlight)
  • Use remark and rehype plugins (math)
• Plugins
• Syntax
• Compatibility
• Architecture
• Appendix A: HTML in markdown
• Appendix B: Components
• Appendix C: line endings in markdown (and JSX)
• Security
• Related
• Contribute
• License

This package is a React component that can be given a string of markdown that it’ll safely render to React elements. You can pass plugins to change how markdown is transformed and pass components that will be used instead of normal HTML elements.

• to learn markdown, see this cheatsheet and tutorial
• to try out react-markdown, see our demo

There are other ways to use markdown in React out there so why use this one? The three main reasons are that they often rely on dangerouslySetInnerHTML, have bugs with how they handle markdown, or don’t let you swap elements for components. react-markdown builds a virtual DOM, so React only replaces what changed, from a syntax tree. That’s supported because we use unified, specifically remark for markdown and rehype for HTML, which are popular tools to transform content with plugins.

This package focusses on making it easy for beginners to safely use markdown in React. When you’re familiar with unified, you can use a modern hooks based alternative react-remark or rehype-react manually. If you instead want to use JavaScript and JSX inside markdown files, use MDX.

This package is ESM only. In Node.js (version 16+), install with npm:

npm install react-markdown

In Deno with esm.sh:

import Markdown from 'https://esm.sh/react-markdown@10'

In browsers with esm.sh:

<script type="module">
  import Markdown from 'https://esm.sh/react-markdown@10?bundle'
</script>

A basic hello world:

import React from 'react'
import {createRoot} from 'react-dom/client'
import Markdown from 'react-markdown'

const markdown = '# Hi, *Pluto*!'

createRoot(document.body).render(<Markdown>{markdown}</Markdown>)

<h1>
  Hi, <em>Pluto</em>!
</h1>

Here is an example that shows how to use a plugin (remark-gfm, which adds support for footnotes, strikethrough, tables, tasklists and URLs directly):

import React from 'react'
import {createRoot} from 'react-dom/client'
import Markdown from 'react-markdown'
import remarkGfm from 'remark-gfm'

const markdown = `Just a link: www.nasa.gov.`

createRoot(document.body).render(
  <Markdown remarkPlugins={[remarkGfm]}>{markdown}</Markdown>
)

<p>
  Just a link: <a href="http://www.nasa.gov">www.nasa.gov</a>.
</p>

This package exports the identifiers MarkdownAsync, MarkdownHooks, and defaultUrlTransform. The default export is Markdown.

It also exports the additional TypeScript types AllowElement, Components, ExtraProps, HooksOptions, Options, and UrlTransform.

Component to render markdown.

This is a synchronous component. When using async plugins, see MarkdownAsync or MarkdownHooks.

• options (Options) — props

React element (ReactElement).

Component to render markdown with support for async plugins through async/await.

Components returning promises are supported on the server. For async support on the client, see MarkdownHooks.

• options (Options) — props

Promise to a React element (Promise<ReactElement>).

Component to render markdown with support for async plugins through hooks.

This uses useEffect and useState hooks. Hooks run on the client and do not immediately render something. For async support on the server, see MarkdownAsync.

• options (Options) — props

React node (ReactNode).

Make a URL safe.

This follows how GitHub works. It allows the protocols http, https, irc, ircs, mailto, and xmpp, and URLs relative to the current protocol (such as /something).

• url (string) — URL

Safe URL (string).

Filter elements (TypeScript type).

• node (Element from hast) — element to check
• index (number | undefined) — index of element in parent
• parent (Node from hast) — parent of element

Whether to allow element (boolean, optional).

Map tag names to components (TypeScript type).

import type {ExtraProps} from 'react-markdown'
import type {ComponentProps, ElementType} from 'react'

type Components = {
  [Key in Extract<ElementType, string>]?: ElementType<ComponentProps<Key> & ExtraProps>
}

Extra fields we pass to components (TypeScript type).

• node (Element from hast, optional) — original node

Configuration for MarkdownHooks (TypeScript type); extends the regular Options with a fallback prop.

Options.

• fallback (ReactNode, optional) — content to render while the processor processing the markdown

Configuration (TypeScript type).

• allowElement (AllowElement, optional) — filter elements; allowedElements / disallowedElements is used first
• allowedElements (Array<string>, default: all tag names) — tag names to allow; cannot combine w/ disallowedElements
• children (string, optional) — markdown
• components (Components, optional) — map tag names to components
• disallowedElements (Array<string>, default: []) — tag names to disallow; cannot combine w/ allowedElements
• rehypePlugins (Array<Plugin>, optional) — list of rehype plugins to use
• remarkPlugins (Array<Plugin>, optional) — list of remark plugins to use
• remarkRehypeOptions (Options from remark-rehype, optional) — options to pass through to remark-rehype
• skipHtml (boolean, default: false) — ignore HTML in markdown completely
• unwrapDisallowed (boolean, default: false) — extract (unwrap) what’s in disallowed elements; normally when say strong is not allowed, it and it’s children are dropped, with unwrapDisallowed the element itself is replaced by its children
• urlTransform (UrlTransform, default: defaultUrlTransform) — change URLs

Transform URLs (TypeScript type).

• url (string) — URL
• key (string, example: 'href') — property name
• node (Element from hast) — element to check

Transformed URL (string, optional).

This example shows how to use a remark plugin. In this case, remark-gfm, which adds support for strikethrough, tables, tasklists and URLs directly:

import React from 'react'
import {createRoot} from 'react-dom/client'
import Markdown from 'react-markdown'
import remarkGfm from 'remark-gfm'

const markdown = `A paragraph with *emphasis* and **strong importance**.

> A block quote with ~strikethrough~ and a URL: https://reactjs.org.

* Lists
* [ ] todo
* [x] done

A table:

| a | b |
| - | - |
`

createRoot(document.body).render(
  <Markdown remarkPlugins={[remarkGfm]}>{markdown}</Markdown>
)

<>
  <p>
    A paragraph with <em>emphasis</em> and <strong>strong importance</strong>.
  </p>
  <blockquote>
    <p>
      A block quote with <del>strikethrough</del> and a URL:{' '}
      <a href="https://reactjs.org">https://reactjs.org</a>.
    </p>
  </blockquote>
  <ul className="contains-task-list">
    <li>Lists</li>
    <li className="task-list-item">
      <input type="checkbox" disabled /> todo
    </li>
    <li className="task-list-item">
      <input type="checkbox" disabled checked /> done
    </li>
  </ul>
  <p>A table:</p>
  <table>
    <thead>
      <tr>
        <th>a</th>
        <th>b</th>
      </tr>
    </thead>
  </table>
</>

This example shows how to use a plugin and give it options. To do that, use an array with the plugin at the first place, and the options second. remark-gfm has an option to allow only double tildes for strikethrough:

import React from 'react'
import {createRoot} from 'react-dom/client'
import Markdown from 'react-markdown'
import remarkGfm from 'remark-gfm'

const markdown = 'This ~is not~ strikethrough, but ~~this is~~!'

createRoot(document.body).render(
  <Markdown remarkPlugins={[[remarkGfm, {singleTilde: false}]]}>
    {markdown}
  </Markdown>
)

<p>
  This ~is not~ strikethrough, but <del>this is</del>!
</p>

This example shows how you can overwrite the normal handling of an element by passing a component. In this case, we apply syntax highlighting with the seriously super amazing react-syntax-highlighter by @conorhastings:

import React from 'react'
import {createRoot} from 'react-dom/client'
import Markdown from 'react-markdown'
import {Prism as SyntaxHighlighter} from 'react-syntax-highlighter'
import {dark} from 'react-syntax-highlighter/dist/esm/styles/prism'

// Did you know you can use tildes instead of backticks for code in markdown? ✨
const markdown = `Here is some JavaScript code:

~~~js
console.log('It works!')
~~~
`

createRoot(document.body).render(
  <Markdown
    children={markdown}
    components={{
      code(props) {
        const {children, className, node, ...rest} = props
        const match = /language-(\w+)/.exec(className || '')
        return match ? (
          <SyntaxHighlighter
            {...rest}
            PreTag="div"
            children={String(children).replace(/\n$/, '')}
            language={match[1]}
            style={dark}
          />
        ) : (
          <code {...rest} className={className}>
            {children}
          </code>
        )
      }
    }}
  />
)

<>
  <p>Here is some JavaScript code:</p>
  <pre>
    <SyntaxHighlighter language="js" style={dark} PreTag="div" children="console.log('It works!')" />
  </pre>
</>

This example shows how a syntax extension (through remark-math) is used to support math in markdown, and a transform plugin (rehype-katex) to render that math.

import React from 'react'
import {createRoot} from 'react-dom/client'
import Markdown from 'react-markdown'
import rehypeKatex from 'rehype-katex'
import remarkMath from 'remark-math'
import 'katex/dist/katex.min.css' // `rehype-katex` does not import the CSS for you

const markdown = `The lift coefficient ($C_L$) is a dimensionless coefficient.`

createRoot(document.body).render(
  <Markdown remarkPlugins={[remarkMath]} rehypePlugins={[rehypeKatex]}>
    {markdown}
  </Markdown>
)

<p>
  The lift coefficient (
  <span className="katex">
    <span className="katex-mathml">
      <math xmlns="http://www.w3.org/1998/Math/MathML">{/* … */}</math>
    </span>
    <span className="katex-html" aria-hidden="true">
      {/* … */}
    </span>
  </span>
  ) is a dimensionless coefficient.
</p>

We use unified, specifically remark for markdown and rehype for HTML, which are tools to transform content with plugins. Here are three good ways to find plugins:

• awesome-remark and awesome-rehype — selection of the most awesome projects
• List of remark plugins and list of rehype plugins — list of all plugins
• remark-plugin and rehype-plugin topics — any tagged repo on GitHub

react-markdown follows CommonMark, which standardizes the differences between markdown implementations, by default. Some syntax extensions are supported through plugins.

We use micromark under the hood for our parsing. See its documentation for more information on markdown, CommonMark, and extensions.

Projects maintained by the unified collective are compatible with maintained versions of Node.js.

When we cut a new major release, we drop support for unmaintained versions of Node. This means we try to keep the current release line, react-markdown@10, compatible with Node.js 16.

They work in all modern browsers (essentially: everything not IE 11). You can use a bundler (such as esbuild, webpack, or Rollup) to use this package in your project, and use its options (or plugins) to add support for legacy browsers.

                                                           react-markdown
         +----------------------------------------------------------------------------------------------------------------+
         |                                                                                                                |
         |  +----------+        +----------------+        +---------------+       +----------------+       +------------+ |
         |  |          |        |                |        |               |       |                |       |            | |
markdown-+->+  remark  +-mdast->+ remark plugins +-mdast->+ remark-rehype +-hast->+ rehype plugins +-hast->+ components +-+->react elements
         |  |          |        |                |        |               |       |                |       |            | |
         |  +----------+        +----------------+        +---------------+       +----------------+       +------------+ |
         |                                                                                                                |
         +----------------------------------------------------------------------------------------------------------------+

To understand what this project does, it’s important to first understand what unified does: please read through the unifiedjs/unified readme (the part until you hit the API section is required reading).

react-markdown is a unified pipeline — wrapped so that most folks don’t need to directly interact with unified. The processor goes through these steps:

• parse markdown to mdast (markdown syntax tree)
• transform through remark (markdown ecosystem)
• transform mdast to hast (HTML syntax tree)
• transform through rehype (HTML ecosystem)
• render hast to React with components

react-markdown typically escapes HTML (or ignores it, with skipHtml) because it is dangerous and defeats the purpose of this library.

However, if you are in a trusted environment (you trust the markdown), and can spare the bundle size (±60kb minzipped), then you can use rehype-raw:

import React from 'react'
import {createRoot} from 'react-dom/client'
import Markdown from 'react-markdown'
import rehypeRaw from 'rehype-raw'

const markdown = `<div class="note">

Some *emphasis* and <strong>strong</strong>!

</div>`

createRoot(document.body).render(
  <Markdown rehypePlugins={[rehypeRaw]}>{markdown}</Markdown>
)

<div className="note">
  <p>
    Some <em>emphasis</em> and <strong>strong</strong>!
  </p>
</div>

Note: HTML in markdown is still bound by how HTML works in CommonMark. Make sure to use blank lines around block-level HTML that again contains markdown!

You can also change the things that come from markdown:

<Markdown
  components={{
    // Map `h1` (`# heading`) to use `h2`s.
    h1: 'h2',
    // Rewrite `em`s (`*like so*`) to `i` with a red foreground color.
    em(props) {
      const {node, ...rest} = props
      return <i style={{color: 'red'}} {...rest} />
    }
  }}
/>

The keys in components are HTML equivalents for the things you write with markdown (such as h1 for # heading). Normally, in markdown, those are: a, blockquote, br, code, em, h1, h2, h3, h4, h5, h6, hr, img, li, ol, p, pre, strong, and ul. With remark-gfm, you can also use del, input, table, tbody, td, th, thead, and tr. Other remark or rehype plugins that add support for new constructs will also work with react-markdown.

The props that are passed are what you probably would expect: an a (link) will get href (and title) props, and img (image) an src, alt and title, etc.

Every component will receive a node. This is the original Element from hast element being turned into a React element.

You might have trouble with how line endings work in markdown and JSX. We recommend the following, which solves all line ending problems:

// If you write actual markdown in your code, put your markdown in a variable;
// **do not indent markdown**:
const markdown = `
# This is perfect!
`

// Pass the value as an expression as an only child:
const result = <Markdown>{markdown}</Markdown>

👆 That works. Read on for what doesn’t and why that is.

You might try to write markdown directly in your JSX and find that it does not work:

<Markdown>
  # Hi

  This is **not** a paragraph.
</Markdown>

The is because in JSX the whitespace (including line endings) is collapsed to a single space. So the above example is equivalent to:

<Markdown> # Hi This is **not** a paragraph. </Markdown>

Instead, to pass markdown to Markdown, you can use an expression: with a template literal:

<Markdown>{`
# Hi

This is a paragraph.
`}</Markdown>

Template literals have another potential problem, because they keep whitespace (including indentation) inside them. That means that the following does not turn into a heading:

<Markdown>{`
    # This is **not** a heading, it’s an indented code block
`}</Markdown>

Use of react-markdown is secure by default. Overwriting urlTransform to something insecure will open you up to XSS vectors. Furthermore, the remarkPlugins, rehypePlugins, and components you use may be insecure.

To make sure the content is completely safe, even after what plugins do, use rehype-sanitize. It lets you define your own schema of what is and isn’t allowed.

• MDX — JSX in markdown
• remark-gfm — add support for GitHub flavored markdown support
• react-remark — hook based alternative
• rehype-react — turn HTML into React elements

See contributing.md in remarkjs/.github for ways to get started. See support.md for ways to get help.

This project has a code of conduct. By interacting with this repository, organization, or community you agree to abide by its terms.

MIT © Espen Hovlandsdal