Change types to work as a type guard

index.test-d.ts

@@ -0,0 +1,22 @@
+import type {Element, Root, RootContent} from 'hast'
+import {expectAssignable, expectType} from 'tsd'
+import {hasProperty} from './index.js'
+
+const aWithTitle = (function (): Root | RootContent {
+  return {type: 'element', tagName: 'a', properties: {title: 'a'}, children: []}
+})()
+
+if (hasProperty(aWithTitle, 'title')) {
+  expectAssignable<Element>(aWithTitle)
+  expectType<
+    Element & {
+      properties: Record<
+        'title',
+        Array<number | string> | number | string | true
+      >
+    }
+  >(aWithTitle)
+  expectType<Array<number | string> | number | string | true>(
+    aWithTitle.properties.title
+  )
+}

lib/index.js

@@ -1,4 +1,5 @@
 /**
+ * @typedef {import('hast').Element} Element
  * @typedef {import('hast').Nodes} Nodes
  */
 
@@ -7,17 +8,20 @@ const own = {}.hasOwnProperty
 /**
  * Check if `node` is an element and has a `name` property.
  *
+ * @template {string} Key
+ *   Type of key.
  * @param {Nodes} node
  *   Node to check (typically `Element`).
- * @param {string} name
+ * @param {Key} name
  *   Property name to check.
- * @returns {boolean}
+ * @returns {node is Element & {properties: Record<Key, Array<number | string> | number | string | true>}}}
  *   Whether `node` is an element that has a `name` property.
+ *
+ *   Note: see <https://github.com/DefinitelyTyped/DefinitelyTyped/blob/27c9274/types/hast/index.d.ts#L37C29-L37C98>.
  */
 export function hasProperty(node, name) {
   const value =
     node.type === 'element' &&
-    node.properties &&
     own.call(node.properties, name) &&
     node.properties[name]
 

package.json

@@ -42,13 +42,14 @@
     "prettier": "^3.0.0",
     "remark-cli": "^11.0.0",
     "remark-preset-wooorm": "^9.0.0",
+    "tsd": "^0.28.0",
     "type-coverage": "^2.0.0",
     "typescript": "^5.0.0",
     "xo": "^0.55.0"
   },
   "scripts": {
     "prepack": "npm run build && npm run format",
-    "build": "tsc --build --clean && tsc --build && type-coverage",
+    "build": "tsc --build --clean && tsc --build && type-coverage && tsd",
     "format": "remark . -qfo && prettier . -w --log-level warn && xo --fix",
     "test-api": "node --conditions development test.js",
     "test-coverage": "c8 --100 --reporter lcov npm run test-api",