fix: add warning for v1 boolean attribute syntax

src/shared/merge.js

@@ -1,7 +1,7 @@
 import deepmerge from 'deepmerge'
 import { findIndex } from '../utils/array'
 import { applyTemplate } from './template'
-import { metaInfoAttributeKeys } from './constants'
+import { metaInfoAttributeKeys, booleanHtmlAttributes } from './constants'
 
 export function arrayMerge({ component, tagIDKeyName, metaTemplateKeyName, contentKeyName }, target, source) {
   // we concat the arrays without merging objects contained in,
@@ -80,6 +80,10 @@ export function merge(target, source, options = {}) {
 
     for (const key in source[attrKey]) {
       if (source[attrKey].hasOwnProperty(key) && source[attrKey][key] === undefined) {
+        if (booleanHtmlAttributes.includes(key)) {
+          // eslint-disable-next-line no-console
+          console.warn('VueMeta: Please note that since v2 the value undefined is not used to indicate boolean attributes anymore, see migration guide for details')
+        }
         delete source[attrKey][key]
       }
     }

test/unit/getMetaInfo.test.js

@@ -820,4 +820,36 @@ describe('getMetaInfo', () => {
       __dangerouslyDisableSanitizersByTagID: {}
     })
   })
+
+  test.only('prints warning for boolean attributes with value undefined', () => {
+    const warn = jest.spyOn(console, 'warn').mockImplementation(() => {})
+
+    const component = new Vue({
+      metaInfo: {
+        htmlAttrs: {
+          amp: undefined
+        }
+      }
+    })
+
+    expect(getMetaInfo(component)).toEqual({
+      title: undefined,
+      titleChunk: '',
+      titleTemplate: '%s',
+      htmlAttrs: {},
+      headAttrs: {},
+      bodyAttrs: {},
+      meta: [],
+      base: [],
+      link: [],
+      style: [],
+      script: [],
+      noscript: [],
+      __dangerouslyDisableSanitizers: [],
+      __dangerouslyDisableSanitizersByTagID: {}
+    })
+
+    expect(warn).toHaveBeenCalledTimes(1)
+    expect(warn).toHaveBeenCalledWith(expect.stringMatching('the value undefined'))
+  })
 })