Add JSDoc based types

.gitignore

@@ -1,5 +1,6 @@
 coverage/
 node_modules/
 .DS_Store
+*.d.ts
 *.log
 yarn.lock

index.js

@@ -1,3 +1,7 @@
+/**
+ * @typedef {import('hast').Root} Root
+ */
+
 import Slugger from 'github-slugger'
 import {hasProperty} from 'hast-util-has-property'
 import {headingRank} from 'hast-util-heading-rank'
@@ -6,16 +10,19 @@ import {visit} from 'unist-util-visit'
 
 const slugs = new Slugger()
 
+/**
+ * Plugin to add `id`s to headings.
+ *
+ * @type {import('unified').Plugin<[], Root>}
+ */
 export default function rehypeSlug() {
-  return transformer
-}
-
-function transformer(tree) {
-  slugs.reset()
+  return (tree) => {
+    slugs.reset()
 
-  visit(tree, 'element', (node) => {
-    if (headingRank(node) && !hasProperty(node, 'id')) {
-      node.properties.id = slugs.slug(toString(node))
-    }
-  })
+    visit(tree, 'element', (node) => {
+      if (headingRank(node) && node.properties && !hasProperty(node, 'id')) {
+        node.properties.id = slugs.slug(toString(node))
+      }
+    })
+  }
 }

package.json

@@ -25,32 +25,41 @@
   ],
   "sideEffects": false,
   "type": "module",
+  "types": "index.d.ts",
   "main": "index.js",
   "files": [
+    "index.d.ts",
     "index.js"
   ],
   "dependencies": {
+    "@types/hast": "^2.0.0",
     "github-slugger": "^1.1.1",
     "hast-util-has-property": "^2.0.0",
     "hast-util-heading-rank": "^2.0.0",
     "hast-util-to-string": "^2.0.0",
+    "unified": "^10.0.0",
     "unist-util-visit": "^4.0.0"
   },
   "devDependencies": {
+    "@types/github-slugger": "^1.3.0",
+    "@types/tape": "^4.0.0",
     "c8": "^7.0.0",
     "prettier": "^2.0.0",
     "rehype": "^12.0.0",
     "remark-cli": "^9.0.0",
     "remark-preset-wooorm": "^8.0.0",
+    "rimraf": "^3.0.0",
     "tape": "^5.0.0",
-    "unified": "^10.0.0",
+    "type-coverage": "^2.0.0",
+    "typescript": "^4.0.0",
     "xo": "^0.39.0"
   },
   "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 format && npm run test-coverage"
+    "test": "npm run build && npm run format && npm run test-coverage"
   },
   "prettier": {
     "tabWidth": 2,
@@ -61,14 +70,17 @@
     "trailingComma": "none"
   },
   "xo": {
-    "prettier": true,
-    "ignores": [
-      "types/"
-    ]
+    "prettier": true
   },
   "remarkConfig": {
     "plugins": [
       "preset-wooorm"
     ]
+  },
+  "typeCoverage": {
+    "atLeast": 100,
+    "detail": true,
+    "strict": true,
+    "ignoreCatch": true
   }
 }

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
+  }
+}