Change to yield undefined, not null

You can still pass it, just that *we* don’t.

index.d.ts

@@ -672,7 +672,7 @@ export type Transformer<
  *   Nothing.
  */
 export type TransformCallback<Tree extends Node = Node> = (
-  error?: Error | null | undefined,
+  error?: Error | undefined,
   node?: Tree | undefined,
   file?: VFile | undefined
 ) => void
@@ -834,7 +834,7 @@ export type CompilerFunction<Tree extends Node = Node, Result = unknown> = (
  *   Nothing.
  */
 export type RunCallback<Tree extends Node = Node> = (
-  error?: Error | null | undefined,
+  error?: Error | undefined,
   node?: Tree | undefined,
   file?: VFile | undefined
 ) => void
@@ -852,7 +852,7 @@ export type RunCallback<Tree extends Node = Node> = (
  *   Nothing.
  */
 export type ProcessCallback<File extends VFile = VFile> = (
-  error?: Error | null | undefined,
+  error?: Error | undefined,
   file?: File | undefined
 ) => void
 

lib/index.js

@@ -98,7 +98,7 @@ function base() {
       }
 
       // Get `key`.
-      return (own.call(namespace, key) && namespace[key]) || null
+      return (own.call(namespace, key) && namespace[key]) || undefined
     }
 
     // Set space.
@@ -314,10 +314,10 @@ function base() {
       return new Promise(executor)
     }
 
-    executor(null, callback)
+    executor(undefined, callback)
 
     /**
-     * @param {((node: Node) => void) | null} resolve
+     * @param {((node: Node) => void) | undefined} resolve
      * @param {(error: Error) => void} reject
      * @returns {void}
      */
@@ -326,7 +326,7 @@ function base() {
       transformers.run(node, vfile(doc), done)
 
       /**
-       * @param {Error | null} error
+       * @param {Error | undefined} error
        * @param {Node} tree
        * @param {VFile} file
        * @returns {void}
@@ -339,7 +339,7 @@ function base() {
           resolve(tree)
         } else {
           // @ts-expect-error: `callback` is defined if `resolve` is not.
-          callback(null, tree, file)
+          callback(undefined, tree, file)
         }
       }
     }
@@ -360,7 +360,7 @@ function base() {
     return result
 
     /**
-     * @param {Error | null} [error]
+     * @param {Error | undefined} [error]
      * @param {Node} [tree]
      * @returns {void}
      */
@@ -385,11 +385,11 @@ function base() {
       return new Promise(executor)
     }
 
-    executor(null, callback)
+    executor(undefined, callback)
 
     /**
-     * @param {((file: VFile) => void) | null} resolve
-     * @param {(error?: Error | null | undefined) => void} reject
+     * @param {((file: VFile) => void) | undefined} resolve
+     * @param {(error?: Error | undefined) => void} reject
      * @returns {void}
      */
     function executor(resolve, reject) {
@@ -402,7 +402,7 @@ function base() {
           /** @type {unknown} */
           const result = processor.stringify(tree, file)
 
-          if (result === undefined || result === null) {
+          if (result === null || result === undefined) {
             // Empty.
           } else if (looksLikeAVFileValue(result)) {
             file.value = result
@@ -415,7 +415,7 @@ function base() {
       })
 
       /**
-       * @param {Error | null | undefined} [error]
+       * @param {Error | undefined} [error]
        * @param {VFile | undefined} [file]
        * @returns {void}
        */
@@ -426,7 +426,7 @@ function base() {
           resolve(file)
         } else {
           // @ts-expect-error: `callback` is defined if `resolve` is not.
-          callback(null, file)
+          callback(undefined, file)
         }
       }
     }
@@ -450,7 +450,7 @@ function base() {
     return file
 
     /**
-     * @param {Error | null | undefined} [error]
+     * @param {Error | undefined} [error]
      * @returns {void}
      */
     function done(error) {

test/data.js

@@ -10,15 +10,15 @@ test('`data`', async function (t) {
   })
 
   await t.test('should yield data as getter (not defined)', async function () {
-    assert.equal(unified().data('foo'), null)
+    assert.equal(unified().data('foo'), undefined)
   })
 
   await t.test('should yield data as getter (defined)', async function () {
     assert.equal(unified().data('foo', 'bar').data('foo'), 'bar')
   })
 
   await t.test('should not yield data prototypal fields', async function () {
-    assert.equal(unified().data('toString'), null)
+    assert.equal(unified().data('toString'), undefined)
   })
 
   await t.test('should yield dataset as getter w/o key', async function () {

test/run.js

@@ -12,7 +12,7 @@ test('`run`', async function (t) {
   await t.test('should pass/yield expected values', async function () {
     await new Promise(function (resolve) {
       unified().run(givenNode, givenFile, function (error, tree, file) {
-        assert.equal(error, null)
+        assert.equal(error, undefined)
         assert.equal(tree, givenNode)
         assert.equal(file, givenFile)
         assert.equal(arguments.length, 3)
@@ -24,7 +24,7 @@ test('`run`', async function (t) {
   await t.test('should pass a file if implicitly not given', async function () {
     await new Promise(function (resolve) {
       unified().run(givenNode, function (error, _, file) {
-        assert.equal(error, null)
+        assert.equal(error, undefined)
         assert.ok(file instanceof VFile)
         resolve(undefined)
       })
@@ -34,7 +34,7 @@ test('`run`', async function (t) {
   await t.test('should pass a file if explicitly not given', async function () {
     await new Promise(function (resolve) {
       unified().run(givenNode, undefined, function (error, _, file) {
-        assert.equal(error, null)
+        assert.equal(error, undefined)
         assert.ok(file instanceof VFile)
         resolve(undefined)
       })
@@ -70,7 +70,7 @@ test('`run`', async function (t) {
             }
           })
           .run(givenNode, function (error, tree) {
-            assert.equal(error, null)
+            assert.equal(error, undefined)
             assert.equal(tree, otherNode)
             resolve(undefined)
           })
@@ -128,7 +128,7 @@ test('`run`', async function (t) {
             }
           })
           .run(givenNode, function (error, tree) {
-            assert.equal(error, null)
+            assert.equal(error, undefined)
             assert.equal(tree, otherNode)
             resolve(undefined)
           })
@@ -150,7 +150,7 @@ test('`run`', async function (t) {
             }
           })
           .run(givenNode, function (error, tree) {
-            assert.equal(error, null)
+            assert.equal(error, undefined)
             assert.equal(tree, otherNode)
             resolve(undefined)
           })
@@ -191,7 +191,7 @@ test('`run`', async function (t) {
             }
           })
           .run(givenNode, function (error, tree) {
-            assert.equal(error, null)
+            assert.equal(error, undefined)
             assert.equal(tree, otherNode)
             resolve(undefined)
           })
@@ -212,7 +212,7 @@ test('`run`', async function (t) {
             }
           })
           .run(givenNode, function (error) {
-            assert.equal(error, null)
+            assert.equal(error, undefined)
             resolve(undefined)
           })
       })
@@ -239,7 +239,7 @@ test('`run`', async function (t) {
             }
           })
           .run(givenNode, function (error) {
-            assert.equal(error, null)
+            assert.equal(error, undefined)
             resolve(undefined)
           })
       })
@@ -277,7 +277,7 @@ test('`run`', async function (t) {
           }
         })
         .run(givenNode, givenFile, function (error, tree, file) {
-          assert.equal(error, null)
+          assert.equal(error, undefined)
           assert.equal(tree, otherNode)
           assert.equal(file, givenFile)
           resolve(undefined)