Answer stability round 1: accept a plain object from another realm

src/adf/document.test.ts

@@ -1,5 +1,6 @@
 import assert from 'node:assert/strict'
 import test from 'node:test'
+import vm from 'node:vm'
 
 import type { JsonValue } from '../json-value.ts'
 import { adfDocumentFault, isAdfDocument } from './document.ts'
@@ -76,6 +77,7 @@ test('accepts the JSON values an attribute may hold', () => {
   assert.equal(isAdfDocument({ content: [{ attrs: { a: [1, 'x', null, true, { b: 2 }] }, type: 'paragraph' }], type: 'doc', version: 1 }), true)
   assert.equal(isAdfDocument({ content: [{ attrs: { a: [() => 1] }, type: 'paragraph' }], type: 'doc', version: 1 }), false)
   assert.equal(isAdfDocument({ content: [{ attrs: { a: Object.assign(Object.create(null), { b: 1 }) }, type: 'paragraph' }], type: 'doc', version: 1 }), true)
+  assert.equal(isAdfDocument(vm.runInNewContext("({ content: [{ attrs: { a: { b: [1] } }, type: 'paragraph' }], type: 'doc', version: 1 })")), true)
 })
 
 test('refuses an attribute value that is an object JSON does not hold', () => {

src/json-value.ts

@@ -19,11 +19,11 @@ export function isJsonValue(value: unknown): value is JsonValue {
   return true
 }
 
-// A Date, Map or class instance passes typeof but JSON writes it as {} or a key per element.
+// Any realm's Object.prototype has a null prototype; a Date, Map or typed array would serialize as {} or a key per element.
 function isPlainObject(value: unknown): value is object {
   if (typeof value !== 'object' || value === null) return false
   const prototype: unknown = Object.getPrototypeOf(value)
-  return prototype === Object.prototype || prototype === null
+  return prototype === null || (typeof prototype === 'object' && Object.getPrototypeOf(prototype) === null)
 }
 
 export function nestingDepth(value: unknown): number {