Change types to base what is returned on tree

If a paragraph is passed, `null` is added to the potential return types.
If a non-paragraph is passed, the input type is returned.

Author: wooorm

.gitignore

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

index.js

@@ -1,22 +1,28 @@
 /**
  * @typedef {import('mdast').Root|import('mdast').Content} Node
+ * @typedef {import('mdast').Paragraph} Paragraph
  */
 
 import {remove} from 'unist-util-remove'
 
 /**
- * @template {Node} T
- * @param {T} tree
- * @returns {T|null}
+ * @template {Node} Tree
+ * @param {Tree} tree
+ * @returns {Tree extends Paragraph ? Tree | null : Tree}
  */
 export function squeezeParagraphs(tree) {
-  return remove(tree, {cascade: false}, (node) =>
+  /**
+   * @param {Node} node
+   * @returns {boolean}
+   */
+  const filter = (node) =>
     Boolean(
       node.type === 'paragraph' &&
         node.children.every(
-          (/** @type {Node} */ node) =>
-            node.type === 'text' && /^\s*$/.test(node.value)
+          (node) => node.type === 'text' && /^\s*$/.test(node.value)
         )
     )
-  )
+
+  // @ts-expect-error: `remove` can’t narrow the above test.
+  return remove(tree, {cascade: false}, filter)
 }

index.test-d.ts

@@ -0,0 +1,17 @@
+import {expectType} from 'tsd'
+import {Root, Heading, Paragraph} from 'mdast'
+import {squeezeParagraphs} from './index.js'
+
+const root: Root = {type: 'root', children: []}
+const paragraph: Paragraph = {type: 'paragraph', children: []}
+const heading: Heading = {type: 'heading', depth: 1, children: []}
+/* eslint-disable @typescript-eslint/consistent-type-assertions */
+const headingOrParagraph = {type: 'paragraph', children: []} as
+  | Heading
+  | Paragraph
+/* eslint-enable @typescript-eslint/consistent-type-assertions */
+
+expectType<Root>(squeezeParagraphs(root))
+expectType<Paragraph | null>(squeezeParagraphs(paragraph))
+expectType<Heading>(squeezeParagraphs(heading))
+expectType<Heading | Paragraph | null>(squeezeParagraphs(headingOrParagraph))

package.json

@@ -46,18 +46,19 @@
     "@types/tape": "^4.0.0",
     "c8": "^7.0.0",
     "prettier": "^2.0.0",
-    "remark-cli": "^9.0.0",
+    "remark-cli": "^10.0.0",
     "remark-preset-wooorm": "^8.0.0",
     "rimraf": "^3.0.0",
     "tape": "^5.0.0",
+    "tsd": "^0.17.0",
     "type-coverage": "^2.0.0",
     "typescript": "^4.0.0",
     "unist-builder": "^3.0.0",
     "xo": "^0.42.0"
   },
   "scripts": {
     "prepack": "npm run build && npm run format",
-    "build": "rimraf \"*.d.ts\" && tsc && type-coverage",
+    "build": "rimraf \"*.d.ts\" && tsc && tsd && type-coverage",
     "format": "remark . -qfo && prettier . -w --loglevel warn && xo --fix",
     "test-api": "node test.js",
     "test-coverage": "c8 --check-coverage --branches 100 --functions 100 --lines 100 --statements 100 --reporter lcov node test.js",