Add option.prefix

Related-to: GH-13.
Closes GH-14.
Closes GH-15.

Reviewed-by: Titus Wormer <tituswormer@gmail.com> 

Co-authored-by: Titus Wormer <tituswormer@gmail.com>

Author: onyxcodes

index.js

@@ -2,6 +2,13 @@
  * @typedef {import('hast').Root} Root
  */
 
+/**
+ * @typedef Options
+ *   Configuration (optional).
+ * @property {string} [prefix='']
+ *   Prefix to add in front of `id`s.
+ */
+
 import Slugger from 'github-slugger'
 import {hasProperty} from 'hast-util-has-property'
 import {headingRank} from 'hast-util-heading-rank'
@@ -13,15 +20,17 @@ const slugs = new Slugger()
 /**
  * Plugin to add `id`s to headings.
  *
- * @type {import('unified').Plugin<Array<void>, Root>}
+ * @type {import('unified').Plugin<[Options?]|Array<void>, Root>}
  */
-export default function rehypeSlug() {
+export default function rehypeSlug(options = {}) {
+  const prefix = options.prefix || ''
+
   return (tree) => {
     slugs.reset()
 
     visit(tree, 'element', (node) => {
       if (headingRank(node) && node.properties && !hasProperty(node, 'id')) {
-        node.properties.id = slugs.slug(toString(node))
+        node.properties.id = prefix + slugs.slug(toString(node))
       }
     })
   }

readme.md

@@ -17,7 +17,7 @@
 *   [Install](#install)
 *   [Use](#use)
 *   [API](#api)
-    *   [`unified().use(rehypeSlug)`](#unifieduserehypeslug)
+    *   [`unified().use(rehypeSlug[, options])`](#unifieduserehypeslug-options)
 *   [Types](#types)
 *   [Compatibility](#compatibility)
 *   [Security](#security)
@@ -117,10 +117,17 @@ Now, running `node example.js` yields:
 This package exports no identifiers.
 The default export is `rehypeSlug`.
 
-### `unified().use(rehypeSlug)`
+### `unified().use(rehypeSlug[, options])`
 
 Add `id`s to headings.
-There are no options.
+
+##### `options`
+
+Configuration (optional).
+
+###### `options.prefix`
+
+Prefix to add in front of `id`s (`string`, default: `''`).
 
 ## Types
 

test.js

@@ -3,7 +3,7 @@ import {rehype} from 'rehype'
 import rehypeSlug from './index.js'
 
 test('rehypeSlug', (t) => {
-  t.plan(2)
+  t.plan(4)
 
   rehype()
     .data('settings', {fragment: true})
@@ -38,4 +38,38 @@ test('rehypeSlug', (t) => {
         )
       }
     )
+
+  rehype()
+    .data('settings', {fragment: true})
+    .use(rehypeSlug, {prefix: 'test-'})
+    .process(
+      [
+        '<section>',
+        '  <h1>Lorem ipsum 😪</h1>',
+        '  <h2>dolor—sit—amet</h2>',
+        '  <h3>consectetur &amp; adipisicing</h3>',
+        '  <h4>elit</h4>',
+        '  <h5>elit</h5>',
+        '  <p>sed</p>',
+        '</section>'
+      ].join('\n'),
+      (error, file) => {
+        t.ifErr(error, 'shouldn’t throw')
+
+        t.equal(
+          String(file),
+          [
+            '<section>',
+            '  <h1 id="test-lorem-ipsum-">Lorem ipsum 😪</h1>',
+            '  <h2 id="test-dolorsitamet">dolor—sit—amet</h2>',
+            '  <h3 id="test-consectetur--adipisicing">consectetur &#x26; adipisicing</h3>',
+            '  <h4 id="test-elit">elit</h4>',
+            '  <h5 id="test-elit-1">elit</h5>',
+            '  <p>sed</p>',
+            '</section>'
+          ].join('\n'),
+          'should match'
+        )
+      }
+    )
 })