Throw more helpful error when setting Pair.commentBefore incorrectly (#…

…157)

src/schema/Pair.js

@@ -37,12 +37,17 @@ export class Pair extends Node {
   }
 
   get commentBefore() {
-    return this.key && this.key.commentBefore
+    return this.key instanceof Node ? this.key.commentBefore : undefined
   }
 
   set commentBefore(cb) {
     if (this.key == null) this.key = new Scalar(null)
-    this.key.commentBefore = cb
+    if (this.key instanceof Node) this.key.commentBefore = cb
+    else {
+      const msg =
+        'Pair.commentBefore is an alias for Pair.key.commentBefore. To set it, the key must be a Node.'
+      throw new Error(msg)
+    }
   }
 
   addToJSMap(ctx, map) {

tests/doc/comments.js

@@ -1,5 +1,6 @@
 import { source } from 'common-tags'
 import { YAML } from '../../src/index'
+import { Pair } from '../../src/schema/Pair'
 
 describe('parse comments', () => {
   describe('body', () => {
@@ -840,3 +841,34 @@ a:
     expect(String(doc)).toBe(src)
   })
 })
+
+describe('Pair.commentBefore', () => {
+  test('Should get key comment', () => {
+    const key = YAML.createNode('foo', true)
+    const pair = new Pair(key, 42)
+    key.commentBefore = 'cc'
+    expect(pair.commentBefore).toBe('cc')
+  })
+
+  test('Should set key comment', () => {
+    const key = YAML.createNode('foo', true)
+    const pair = new Pair(key, 42)
+    pair.commentBefore = 'cc'
+    expect(key.commentBefore).toBe('cc')
+  })
+
+  test('Should create a key from a null value', () => {
+    const pair = new Pair(null, 42)
+    expect(pair.key).toBeNull()
+    pair.commentBefore = 'cc'
+    expect(pair.key).not.toBeNull()
+    expect(pair.key.commentBefore).toBe('cc')
+  })
+
+  test('Should throw for non-Node key', () => {
+    const pair = new Pair({ foo: 'bar' }, 42)
+    expect(() => {
+      pair.commentBefore = 'cc'
+    }).toThrow(/commentBefore is an alias/)
+  })
+})