Add mention in docs for previous (#173)

docs/05_content_nodes.md

@@ -167,7 +167,7 @@ String(doc)
 
 #### `YAML.Document#createNode(value, options?): Node`
 
-To create a new node, use the `createNode(value, options?)` document method. This will recursively wrap any input with appropriate `Node` containers. Generic objects as well as `Map` and its descendants become mappings, while arrays and other iterable objects result in sequences.
+To create a new node, use the `createNode(value, options?)` document method. This will recursively wrap any input with appropriate `Node` containers. Generic JS `Object` values as well as `Map` and its descendants become mappings, while arrays and other iterable objects result in sequences. With `Object`, entries that have an `undefined` value are dropped.
 
 To specify the collection type, set `options.tag` to its identifying string, e.g. `"!!omap"`. Note that this requires the corresponding tag to be available in the document's schema. If `options.wrapScalars` is undefined or `true`, plain values are wrapped in `Scalar` objects.
 

tests/doc/stringify.js

@@ -727,7 +727,13 @@ describe('undefined values', () => {
 
   test("{ a: 'A', b: undefined, c: 'C' }", () => {
     expect(YAML.stringify({ a: 'A', b: undefined, c: 'C' })).toBe(
-      'a: A\nc: C\n'
+      'a: A\nc: C\n' // note: No `b` key
+    )
+  })
+
+  test("{ a: 'A', b: null, c: 'C' }", () => {
+    expect(YAML.stringify({ a: 'A', b: null, c: 'C' })).toBe(
+      'a: A\nb: null\nc: C\n'
     )
   })