Add 'empty' style option for !!null

fix #570

CHANGELOG.md

@@ -25,6 +25,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added `.mjs` (es modules) support.
 - Added `quotingType` and `forceQuotes` options for dumper to configure
   string literal style, #290, #529.
+- Added `styles: { '!!null': 'empty' }` option for dumper
+  (serializes `{ foo: null }` as "`foo: `"), #570.
 
 ### Fixed
 - Astral characters are no longer encoded by dump/safeDump, #587.

lib/type/null.js

@@ -28,7 +28,8 @@ module.exports = new Type('tag:yaml.org,2002:null', {
     canonical: function () { return '~';    },
     lowercase: function () { return 'null'; },
     uppercase: function () { return 'NULL'; },
-    camelcase: function () { return 'Null'; }
+    camelcase: function () { return 'Null'; },
+    empty:     function () { return '';     }
   },
   defaultStyle: 'lowercase'
 });

test/issues/0570.js

@@ -0,0 +1,24 @@
+'use strict';
+
+
+const assert = require('assert');
+const yaml = require('../../');
+
+
+it('should dump null in different styles', function () {
+  let dump, src = { foo: null, bar: 1 };
+
+  let tests = {
+    lowercase: 'null',
+    uppercase: 'NULL',
+    camelcase: 'Null',
+    canonical: '~',
+    empty: ''
+  };
+
+  for (let [ name, value ] of Object.entries(tests)) {
+    dump = yaml.dump(src, { styles: { '!!null': name } });
+    assert.strictEqual(dump, 'foo: ' + value + '\nbar: 1\n');
+    assert.deepStrictEqual(yaml.load(dump), src);
+  }
+});