diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..c6c8b36
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,9 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 2
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
diff --git a/.github/workflows/bb.yml b/.github/workflows/bb.yml
new file mode 100644
index 0000000..0198fc3
--- /dev/null
+++ b/.github/workflows/bb.yml
@@ -0,0 +1,13 @@
+name: bb
+on:
+  issues:
+    types: [opened, reopened, edited, closed, labeled, unlabeled]
+  pull_request_target:
+    types: [opened, reopened, edited, closed, labeled, unlabeled]
+jobs:
+  main:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: unifiedjs/beep-boop-beta@main
+        with:
+          repo-token: ${{secrets.GITHUB_TOKEN}}
diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
new file mode 100644
index 0000000..fe284ad
--- /dev/null
+++ b/.github/workflows/main.yml
@@ -0,0 +1,21 @@
+name: main
+on:
+  - pull_request
+  - push
+jobs:
+  main:
+    name: ${{matrix.node}}
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dcodeIO/setup-node-nvm@master
+        with:
+          node-version: ${{matrix.node}}
+      - run: npm install
+      - run: npm test
+      - uses: codecov/codecov-action@v1
+    strategy:
+      matrix:
+        node:
+          - lts/erbium
+          - node
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..53a29e3
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,6 @@
+coverage/
+node_modules/
+.DS_Store
+*.d.ts
+*.log
+yarn.lock
diff --git a/.npmrc b/.npmrc
new file mode 100644
index 0000000..43c97e7
--- /dev/null
+++ b/.npmrc
@@ -0,0 +1 @@
+package-lock=false
diff --git a/.prettierignore b/.prettierignore
new file mode 100644
index 0000000..cebe81f
--- /dev/null
+++ b/.prettierignore
@@ -0,0 +1,2 @@
+coverage/
+*.md
diff --git a/index.js b/index.js
new file mode 100644
index 0000000..f6f854d
--- /dev/null
+++ b/index.js
@@ -0,0 +1,85 @@
+/**
+ * @typedef {import('hast').Root} Root
+ * @typedef {import('hast').Properties} Properties
+ * @typedef {import('hast').Element['children'][number]} ElementChild
+ *
+ * @typedef Options
+ *   Configuration.
+ * @property {'_self'|'_blank'|'_parent'|'_top'|false} [target='_blank']
+ *   How to display referenced documents (`string?`: `_self`, `_blank`,
+ *   `_parent`, or `_top`, default: `_blank`).
+ *   Pass `false` to not set `target`s on links.
+ * @property {string[]|string|false} [rel=['nofollow', 'noopener', 'noreferrer']]
+ *   Link types to hint about the referenced documents.
+ *   Pass `false` to not set `rel`s on links.
+ *
+ *   **Note**: when using a `target`, add `noopener` and `noreferrer` to avoid
+ *   exploitation of the `window.opener` API.
+ * @property {string[]} [protocols=['http', 'https']]
+ *   Protocols to check, such as `mailto` or `tel`.
+ * @property {ElementChild|ElementChild[]} [content]
+ *   hast content to insert at the end of external links.
+ *   Will be inserted in a `<span>` element.
+ *
+ *   Useful for improving accessibility by giving users advanced warning when
+ *   opening a new window.
+ * @property {Properties} [contentProperties]
+ *   hast properties to add to the `span` wrapping `content`, when given.
+ */
+
+import {visit} from 'unist-util-visit'
+import {parse} from 'space-separated-tokens'
+import absolute from 'is-absolute-url'
+import extend from 'extend'
+
+const defaultTarget = '_blank'
+const defaultRel = ['nofollow', 'noopener', 'noreferrer']
+const defaultProtocols = ['http', 'https']
+
+/**
+ * Plugin to automatically add `target` and `rel` attributes to external links.
+ *
+ * @type {import('unified').Plugin<[Options?] | void[], Root>}
+ */
+export default function rehypeExternalLinks(options = {}) {
+  const target = options.target
+  const rel = typeof options.rel === 'string' ? parse(options.rel) : options.rel
+  const protocols = options.protocols || defaultProtocols
+  const content =
+    options.content && !Array.isArray(options.content)
+      ? [options.content]
+      : options.content
+  const contentProperties = options.contentProperties || {}
+
+  return (tree) => {
+    visit(tree, 'element', (node) => {
+      if (
+        node.tagName === 'a' &&
+        node.properties &&
+        typeof node.properties.href === 'string'
+      ) {
+        const url = node.properties.href
+        const protocol = url.slice(0, url.indexOf(':'))
+
+        if (absolute(url) && protocols.includes(protocol)) {
+          if (target !== false) {
+            node.properties.target = target || defaultTarget
+          }
+
+          if (rel !== false) {
+            node.properties.rel = (rel || defaultRel).concat()
+          }
+
+          if (content) {
+            node.children.push({
+              type: 'element',
+              tagName: 'span',
+              properties: extend(true, contentProperties),
+              children: extend(true, content)
+            })
+          }
+        }
+      }
+    })
+  }
+}
diff --git a/license b/license
new file mode 100644
index 0000000..8d8660d
--- /dev/null
+++ b/license
@@ -0,0 +1,22 @@
+(The MIT License)
+
+Copyright (c) 2016 Titus Wormer <tituswormer@gmail.com>
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+'Software'), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
diff --git a/package.json b/package.json
new file mode 100644
index 0000000..d735e29
--- /dev/null
+++ b/package.json
@@ -0,0 +1,86 @@
+{
+  "name": "rehype-external-links",
+  "version": "0.0.0",
+  "description": "rehype plugin to automatically add `target` and `rel` attributes to external links",
+  "license": "MIT",
+  "keywords": [
+    "unified",
+    "rehype",
+    "rehype-plugin",
+    "plugin",
+    "hast",
+    "html",
+    "markdown",
+    "external",
+    "link",
+    "url"
+  ],
+  "repository": "rehypejs/rehype-external-links",
+  "bugs": "https://github.com/rehypejs/rehype-external-links/issues",
+  "funding": {
+    "type": "opencollective",
+    "url": "https://opencollective.com/unified"
+  },
+  "author": "Titus Wormer <tituswormer@gmail.com> (https://wooorm.com)",
+  "contributors": [
+    "Titus Wormer <tituswormer@gmail.com> (https://wooorm.com)"
+  ],
+  "sideEffects": false,
+  "type": "module",
+  "main": "index.js",
+  "types": "index.d.ts",
+  "files": [
+    "index.d.ts",
+    "index.js"
+  ],
+  "dependencies": {
+    "@types/hast": "^2.0.0",
+    "extend": "^3.0.0",
+    "is-absolute-url": "^4.0.0",
+    "space-separated-tokens": "^2.0.0",
+    "unified": "^10.0.0",
+    "unist-util-visit": "^4.0.0"
+  },
+  "devDependencies": {
+    "@types/tape": "^4.0.0",
+    "c8": "^7.0.0",
+    "prettier": "^2.0.0",
+    "rehype": "^12.0.0",
+    "remark-cli": "^10.0.0",
+    "remark-preset-wooorm": "^9.0.0",
+    "rimraf": "^3.0.0",
+    "tape": "^5.0.0",
+    "type-coverage": "^2.0.0",
+    "typescript": "^4.0.0",
+    "xo": "^0.44.0"
+  },
+  "scripts": {
+    "build": "rimraf \"*.d.ts\" && tsc && type-coverage",
+    "format": "remark . -qfo && prettier . -w --loglevel warn && xo --fix",
+    "test-api": "node --conditions development test.js",
+    "test-coverage": "c8 --check-coverage --branches 100 --functions 100 --lines 100 --statements 100 --reporter lcov npm run test-api",
+    "test": "npm run build && npm run format && npm run test-coverage"
+  },
+  "prettier": {
+    "tabWidth": 2,
+    "useTabs": false,
+    "singleQuote": true,
+    "bracketSpacing": false,
+    "semi": false,
+    "trailingComma": "none"
+  },
+  "xo": {
+    "prettier": true
+  },
+  "remarkConfig": {
+    "plugins": [
+      "preset-wooorm"
+    ]
+  },
+  "typeCoverage": {
+    "atLeast": 100,
+    "detail": true,
+    "strict": true,
+    "ignoreCatch": true
+  }
+}
diff --git a/readme.md b/readme.md
new file mode 100644
index 0000000..b564969
--- /dev/null
+++ b/readme.md
@@ -0,0 +1,182 @@
+# rehype-external-links
+
+[![Build][build-badge]][build]
+[![Coverage][coverage-badge]][coverage]
+[![Downloads][downloads-badge]][downloads]
+[![Size][size-badge]][size]
+[![Sponsors][sponsors-badge]][collective]
+[![Backers][backers-badge]][collective]
+[![Chat][chat-badge]][chat]
+
+[**rehype**][rehype] plugin to automatically add `target` and `rel` attributes
+to external links.
+
+## Install
+
+This package is [ESM only](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c):
+Node 12+ is needed to use it and it must be `import`ed instead of `require`d.
+
+[npm][]:
+
+```sh
+npm install rehype-external-links
+```
+
+## Use
+
+Say we have the following module, `example.js`:
+
+```js
+import {unified} from 'unified'
+import remarkParse from 'remark-parse'
+import remarkRehype from 'remark-rehype'
+import rehypeExternalLinks from 'rehype-external-links'
+import rehypeStringify from 'rehype-stringify'
+
+const input = '[rehype](https://github.com/rehypejs/rehype)'
+
+unified()
+  .use(remarkParse)
+  .use(remarkRehype)
+  .use(rehypeExternalLinks, {target: false, rel: ['nofollow']})
+  .use(rehypeStringify)
+  .process(input)
+  .then((file) => {
+    console.log(String(file))
+  })
+```
+
+Now, running `node example` yields:
+
+```html
+<p><a href="https://github.com/rehypejs/rehype" rel="nofollow">rehype</a></p>
+```
+
+## API
+
+This package exports no identifiers.
+The default export is `rehypeExternalLinks`.
+
+### `unified().use(rehypeExternalLinks[, options])`
+
+Automatically add `target` and `rel` attributes to external links.
+
+##### `options`
+
+###### `options.target`
+
+How to display referenced documents (`string?`: `_self`, `_blank`, `_parent`,
+or `_top`, default: `_blank`).
+Pass `false` to not set `target`s on links.
+
+###### `options.rel`
+
+[Link types][mdn-rel] to hint about the referenced documents (`Array.<string>`
+or `string`, default: `['nofollow', 'noopener', 'noreferrer']`).
+Pass `false` to not set `rel`s on links.
+
+**Note**: when using a `target`, add [`noopener` and `noreferrer`][mdn-a] to
+avoid exploitation of the `window.opener` API.
+
+###### `options.protocols`
+
+Protocols to check, such as `mailto` or `tel` (`Array.<string>`, default:
+`['http', 'https']`).
+
+###### `options.content`
+
+[**hast**][hast] content to insert at the end of external links
+([**Node**][node] or [**Children**][children]).
+Will be inserted in a `<span>` element.
+
+Useful for improving accessibility by [giving users advanced warning when
+opening a new window][g201].
+
+###### `options.contentProperties`
+
+[`Properties`][properties] to add to the `span` wrapping `content`, when
+given.
+
+## Security
+
+Improper use of `rehype-external-links` can open you up to a
+[cross-site scripting (XSS)][xss] attack.
+
+Either do not combine this plugin with user content or use
+[`rehype-sanitize`][sanitize].
+
+## Contribute
+
+See [`contributing.md`][contributing] in [`rehypejs/.github`][health] for ways
+to get started.
+See [`support.md`][support] for ways to get help.
+
+This project has a [code of conduct][coc].
+By interacting with this repository, organization, or community you agree to
+abide by its terms.
+
+## License
+
+[MIT][license] © [Titus Wormer][author]
+
+<!-- Definitions -->
+
+[build-badge]: https://github.com/rehypejs/rehype-external-links/workflows/main/badge.svg
+
+[build]: https://github.com/rehypejs/rehype-external-links/actions
+
+[coverage-badge]: https://img.shields.io/codecov/c/github/rehypejs/rehype-external-links.svg
+
+[coverage]: https://codecov.io/github/rehypejs/rehype-external-links
+
+[downloads-badge]: https://img.shields.io/npm/dm/rehype-external-links.svg
+
+[downloads]: https://www.npmjs.com/package/rehype-external-links
+
+[size-badge]: https://img.shields.io/bundlephobia/minzip/rehype-external-links.svg
+
+[size]: https://bundlephobia.com/result?p=rehype-external-links
+
+[sponsors-badge]: https://opencollective.com/unified/sponsors/badge.svg
+
+[backers-badge]: https://opencollective.com/unified/backers/badge.svg
+
+[collective]: https://opencollective.com/unified
+
+[chat-badge]: https://img.shields.io/badge/chat-discussions-success.svg
+
+[chat]: https://github.com/rehypejs/rehype/discussions
+
+[npm]: https://docs.npmjs.com/cli/install
+
+[health]: https://github.com/rehypejs/.github
+
+[contributing]: https://github.com/rehypejs/.github/blob/HEAD/contributing.md
+
+[support]: https://github.com/rehypejs/.github/blob/HEAD/support.md
+
+[coc]: https://github.com/rehypejs/.github/blob/HEAD/code-of-conduct.md
+
+[license]: license
+
+[author]: https://wooorm.com
+
+[rehype]: https://github.com/rehypejs/rehype
+
+[xss]: https://en.wikipedia.org/wiki/Cross-site_scripting
+
+[sanitize]: https://github.com/rehypejs/rehype-sanitize
+
+[mdn-rel]: https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types
+
+[mdn-a]: https://developer.mozilla.org/en/docs/Web/HTML/Element/a
+
+[hast]: https://github.com/syntax-tree/hast
+
+[properties]: https://github.com/syntax-tree/hast#properties
+
+[node]: https://github.com/syntax-tree/hast#nodes
+
+[children]: https://github.com/syntax-tree/unist#child
+
+[g201]: https://www.w3.org/WAI/WCAG21/Techniques/general/G201
diff --git a/test.js b/test.js
new file mode 100644
index 0000000..2a6ea0a
--- /dev/null
+++ b/test.js
@@ -0,0 +1,199 @@
+import test from 'tape'
+import {rehype} from 'rehype'
+import rehypeExternalLinks from './index.js'
+
+test('rehypeExternalLinks', async (t) => {
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks)
+        .process('<a href="./example.html">relative</a>')
+    ),
+    '<a href="./example.html">relative</a>',
+    'should not change a relative link'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks)
+        .process('<a href="#example">fragment</a>')
+    ),
+    '<a href="#example">fragment</a>',
+    'should not change a fragment link'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks)
+        .process('<a href="?search">search</a>')
+    ),
+    '<a href="?search">search</a>',
+    'should not change a search link'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks)
+        .process('<a href="mailto:a@b.com">mailto</a>')
+    ),
+    '<a href="mailto:a@b.com">mailto</a>',
+    'should not change a mailto link'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks)
+        .process('<a href="http://example.com">http</a>')
+    ),
+    '<a href="http://example.com" target="_blank" rel="nofollow noopener noreferrer">http</a>',
+    'should change a http link'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks)
+        .process('<a href="https://example.com">https</a>')
+    ),
+    '<a href="https://example.com" target="_blank" rel="nofollow noopener noreferrer">https</a>',
+    'should change a https link'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks)
+        .process('<a href="www.example.com">www</a>')
+    ),
+    '<a href="www.example.com">www</a>',
+    'should not change a www link'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks, {target: false})
+        .process('<a href="http://example.com">http</a>')
+    ),
+    '<a href="http://example.com" rel="nofollow noopener noreferrer">http</a>',
+    'should not add a `[target]` w/ `target: false`'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks, {rel: false})
+        .process('<a href="http://example.com">http</a>')
+    ),
+    '<a href="http://example.com" target="_blank">http</a>',
+    'should not add a `[rel]` w/ `rel: false`'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks, {target: '_parent', rel: false})
+        .process('<a href="http://example.com">http</a>')
+    ),
+    '<a href="http://example.com" target="_parent">http</a>',
+    'should not add a `[target]` w/ `target` set to a known target'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks, {target: false, rel: 'nofollow'})
+        .process('<a href="http://example.com">http</a>')
+    ),
+    '<a href="http://example.com" rel="nofollow">http</a>',
+    'should not add a `[rel]` w/ `rel` set to a string'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks, {target: false, rel: ['nofollow']})
+        .process('<a href="http://example.com">http</a>')
+    ),
+    '<a href="http://example.com" rel="nofollow">http</a>',
+    'should not add a `[rel]` w/ `rel` set to an array'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks, {protocols: ['mailto']})
+        .process('<a href="mailto:a@b.com">mailto</a>')
+    ),
+    '<a href="mailto:a@b.com" target="_blank" rel="nofollow noopener noreferrer">mailto</a>',
+    'should support `mailto` protocols w/ `mailto` in `protocols`'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks, {
+          content: {type: 'text', value: ' (opens in a new window)'}
+        })
+        .process('<a href="http://example.com">http</a>')
+    ),
+    '<a href="http://example.com" target="_blank" rel="nofollow noopener noreferrer">http<span> (opens in a new window)</span></a>',
+    'should add content at the end of the link w/ `content` as a single child'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks, {
+          content: [
+            {type: 'text', value: ' ('},
+            {
+              type: 'element',
+              tagName: 'em',
+              properties: {},
+              children: [{type: 'text', value: 'opens in a new window'}]
+            },
+            {type: 'text', value: ')'}
+          ]
+        })
+        .process('<a href="http://example.com">http</a>')
+    ),
+    '<a href="http://example.com" target="_blank" rel="nofollow noopener noreferrer">http<span> (<em>opens in a new window</em>)</span></a>',
+    'should add content at the end of the link w/ `content` as an array of children'
+  )
+
+  t.equal(
+    String(
+      await rehype()
+        .use({settings: {fragment: true}})
+        .use(rehypeExternalLinks, {
+          contentProperties: {className: ['alpha', 'bravo']},
+          content: {type: 'text', value: ' (opens in a new window)'}
+        })
+        .process('<a href="http://example.com">http</a>')
+    ),
+    '<a href="http://example.com" target="_blank" rel="nofollow noopener noreferrer">http<span class="alpha bravo"> (opens in a new window)</span></a>',
+    'should add properties to the span at the end of the link w/ `contentProperties`'
+  )
+
+  t.end()
+})
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..e31adf8
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,16 @@
+{
+  "include": ["*.js"],
+  "compilerOptions": {
+    "target": "ES2020",
+    "lib": ["ES2020"],
+    "module": "ES2020",
+    "moduleResolution": "node",
+    "allowJs": true,
+    "checkJs": true,
+    "declaration": true,
+    "emitDeclarationOnly": true,
+    "allowSyntheticDefaultImports": true,
+    "skipLibCheck": true,
+    "strict": true
+  }
+}