Change to pass undefined, not null

Author: wooorm

lib/index.js

@@ -122,7 +122,7 @@
  *   Parent of the section.
  * @property {number} start
  *   Index of `start` in `parent`.
- * @property {number | null} end
+ * @property {number | undefined} end
  *   Index of `end` in `parent`.
  *
  * @typedef Options
@@ -151,7 +151,6 @@
 import {ok as assert} from 'devlop'
 import {toString} from 'mdast-util-to-string'
 
-// To do: next major: remove `null` in API output.
 // To do: next major: remove `value` parameter in `TestFunction`?
 // Add support for `unist-util-is`?
 
@@ -255,7 +254,7 @@ export function headingRange(tree, options, handler) {
     const nodes = handler(head, from, children[end], {
       parent,
       start,
-      end: children[end] ? end : null
+      end: children[end] ? end : undefined
     })
 
     if (nodes) {

readme.md

@@ -183,7 +183,7 @@ Extra info (TypeScript type).
     — parent of the section
 *   `start` (`number`)
     — index of `start` in `parent`
-*   `end` (`number` or `null`)
+*   `end` (`number` or `undefined`)
     — index of `end` in `parent`
 
 ### `Options`