Change to return undefined, not null

lib/index.js

@@ -18,18 +18,17 @@
  *   Heading node to check.
  * @param {Style | null | undefined} [relative]
  *   Relative style (preferred style).
- * @returns {Style | null}
- *   Style, if it can be inferred, `null` otherwise.
+ * @returns {Style | undefined}
+ *   Style, if it can be inferred, `undefined` otherwise.
  */
-// To do: next major: use `undefined`.
 export function headingStyle(node, relative) {
   const last = node.children[node.children.length - 1]
   const depth = node.depth
   const pos = node.position && node.position.end
   const final = last && last.position && last.position.end
 
   if (!pos) {
-    return null
+    return undefined
   }
 
   // This can only occur for `'atx'` and `'atx-closed'` headings.
@@ -59,12 +58,12 @@ export function headingStyle(node, relative) {
  *
  * @param {number} depth
  * @param {Style | null | undefined} relative
- * @returns {Style | null}
+ * @returns {Style | undefined}
  */
 function consolidate(depth, relative) {
   return depth < 3
     ? 'atx'
     : relative === 'atx' || relative === 'setext'
     ? relative
-    : null
+    : undefined
 }

readme.md

@@ -77,7 +77,7 @@ headingStyle(fromMarkdown('# ATX').children[0]) // => 'atx'
 headingStyle(fromMarkdown('# ATX #\n').children[0]) // => 'atx-closed'
 headingStyle(fromMarkdown('ATX\n===').children[0]) // => 'setext'
 
-headingStyle(fromMarkdown('### ATX').children[0]) // => null
+headingStyle(fromMarkdown('### ATX').children[0]) // => undefined
 headingStyle(fromMarkdown('### ATX').children[0], 'setext') // => 'setext'
 ```
 
@@ -103,7 +103,7 @@ considered setext.
 
 ###### Returns
 
-Style ([`Style`][api-style]) if it can be inferred, `null` otherwise.
+Style ([`Style`][api-style]) if it can be inferred, `undefined` otherwise.
 
 ### `Style`
 

test.js

@@ -28,7 +28,7 @@ test('headingStyle', async function (t) {
         type: 'heading',
         children: [{type: 'text', value: 'foo'}]
       }),
-      null
+      undefined
     )
   })
 
@@ -45,7 +45,7 @@ test('headingStyle', async function (t) {
   })
 
   await t.test('should work on ambiguous nodes', async function () {
-    assert.equal(headingStyle(parseFirstNode('### ATX')), null)
+    assert.equal(headingStyle(parseFirstNode('### ATX')), undefined)
   })
 
   await t.test(