When creating a mapping from a JS Object, drop undefined values (Closes

#173)

BREAKING CHANGE: When creating a YAML mapping from a JS Object, keys
with `undefined` values are not included. This matches the behaviour of
`JSON.stringify()`. In all other cases, the handling of `undefined` is
unchanged, so e.g. `[1, undefined, 2]` will still result in a sequence
of three items, the second of which has a `null` value. This too matches
the behaviour of `JSON.stringify()`.

Previously, `undefined` object values were mapped to `null`.

src/tags/failsafe/map.js

@@ -7,8 +7,10 @@ function createMap(schema, obj, ctx) {
   if (obj instanceof Map) {
     for (const [key, value] of obj) map.items.push(createPair(key, value, ctx))
   } else if (obj && typeof obj === 'object') {
-    for (const key of Object.keys(obj))
-      map.items.push(createPair(key, obj[key], ctx))
+    for (const key of Object.keys(obj)) {
+      const value = obj[key]
+      if (value !== undefined) map.items.push(createPair(key, value, ctx))
+    }
   }
   if (typeof schema.sortMapEntries === 'function') {
     map.items.sort(schema.sortMapEntries)

tests/doc/stringify.js

@@ -2,7 +2,7 @@
 
 import { source } from 'common-tags'
 import YAML from '../../index.js'
-import { Pair } from '../../types.js'
+import { Pair, Scalar } from '../../types.js'
 import { Type, stringifyString } from '../../util.js'
 
 for (const [name, version] of [
@@ -715,3 +715,33 @@ describe('Document markers in top-level scalars', () => {
     expect(YAML.parse(str)).toBe('foo\n%bar\n')
   })
 })
+
+describe('undefined values', () => {
+  test('undefined', () => {
+    expect(YAML.stringify(undefined)).toBe('\n')
+  })
+
+  test('[1, undefined, 2]', () => {
+    expect(YAML.stringify([1, undefined, 2])).toBe('- 1\n- null\n- 2\n')
+  })
+
+  test("{ a: 'A', b: undefined, c: 'C' }", () => {
+    expect(YAML.stringify({ a: 'A', b: undefined, c: 'C' })).toBe(
+      'a: A\nc: C\n'
+    )
+  })
+
+  test("{ a: 'A', b: Scalar(undefined), c: 'C' }", () => {
+    const obj = { a: 'A', b: new Scalar(undefined), c: 'C' }
+    expect(YAML.stringify(obj)).toBe('a: A\nb: null\nc: C\n')
+  })
+
+  test("Map { 'a' => 'A', 'b' => undefined, 'c' => 'C' }", () => {
+    const map = new Map([
+      ['a', 'A'],
+      ['b', undefined],
+      ['c', 'C']
+    ])
+    expect(YAML.stringify(map)).toBe('a: A\nb: null\nc: C\n')
+  })
+})