add heading slugs

src/lib/Post.svelte

@@ -1,5 +1,6 @@
 <script context="module">
     import { formatDate } from './datefmt.js';
+    import { makeSlug } from '$lib/slug.js';
     import Link from './Link.svelte';
     export { Link as a };
 </script>
@@ -13,7 +14,7 @@
 </svelte:head>
 
 <div id="post">
-    <h1>{title}</h1>
+    <h1 id="{makeSlug(title)}">{title}</h1>
     <p><em>{formatDate(date)}</em></p>
     <slot></slot>
 </div>

src/lib/slug.js

@@ -1,10 +1,50 @@
-// this approach won't work if an attribute value contains ">"
-const tag = /<[^>]*?>/g
-const nonAlphaNum = /[^A-Za-z0-9\-]/g
+const nonAlphaNum = /[^A-Za-z0-9\-]/g;
 const space = /\s/g
 
-export default function slugify(text) {
-    return text.replace(tag, '')
+export function makeSlug(text) {
+    return text
+        .toLowerCase()
         .replace(space, '-')
-        .replace(nonAlphNum, '')
-}
+        .replace(nonAlphaNum, '')
+}
+
+function apply(node, types, fn) {
+    if (typeof types === 'string') {
+        types = new Set([types]);
+    }
+    else if (!(types instanceof Set)) {
+        types = new Set(types)
+        console.log(types)
+    }
+
+    if (types.has(node.type)) {
+        fn(node);
+    }
+    if ('children' in node) {
+        for (let child of node.children) {
+            apply(child, types, fn);
+        }
+    }
+}
+
+function getTextContent(node) {
+    let segments = [];
+    apply(node, 'text', textNode => {
+        // skip all-whitespace strings
+        if (textNode.value.match(/^\s+$/)) return;
+        segments.push(textNode.value.trim());
+    });
+
+    return segments.join(' ');
+}
+
+export default function slug() {
+    return (tree) => {
+        apply(tree, 'element', e => {
+            if (e.tagName.match(/h[1-6]/)) {
+                let text = getTextContent(e);
+                e.properties.id = makeSlug(text);
+            }
+        })
+    }
+}

svelte.config.js

@@ -1,10 +1,14 @@
 import { mdsvex } from 'mdsvex';
 import staticAdapter from '@sveltejs/adapter-static';
+import slug from './src/lib/slug.js';
 
 
 const config = {
 	extensions: ['.svelte', '.svx'],
-	preprocess: mdsvex({layout: './src/lib/Post.svelte'}),
+	preprocess: mdsvex({
+		layout: './src/lib/Post.svelte',
+		rehypePlugins: [slug],
+	}),
 	kit: {
 		// hydrate the <div id="svelte"> element in src/app.html
 		target: '#svelte',