add info about Jest in readme

README.md

@@ -33,8 +33,9 @@ Use this package to validate and parse AsyncAPI documents —either YAML or JSON
 - [Circular references](#circular-references)
 - [Stringify](#stringify)
 - [Convert to the old API](#convert-to-the-old-api)
-- [Bundler configuration](#bundler-configuration)
-  * [Webpack](#webpack)
+- [Notes](#notes)
+  * [Using with Webpack](#using-with-webpack)
+  * [Testing with [Jest](https://jestjs.io/)](#testing-with-jesthttpsjestjsio)
 - [Develop](#develop)
 - [Contributing](#contributing)
 - [Contributors](#contributors)
@@ -385,9 +386,9 @@ const oldAsyncAPIDocument = convertToOldAPI(document);
 > **Warning**
 > The old api will be supported only for a certain period of time. The target date for turning off support of the old API is around the end of January 2023.
 
-## Bundler configuration
+## Notes
 
-### Webpack
+### Using with Webpack
 
 Versions `<5` of Webpack should handle bundling without problems. Due to the fact that Webpack 5 no longer does fallbacks to native NodeJS modules by default we need to install `buffer` package and add fallbacks:
 
@@ -404,6 +405,23 @@ Versions `<5` of Webpack should handle bundling without problems. Due to the fac
 }
 ```
 
+### Testing with [Jest](https://jestjs.io/)
+
+Using a Parser in an application that is tested using [Jest](https://jestjs.io/), there will probably an error like: 
+
+```bash
+Cannot find module 'nimma/legacy' from 'node_modules/@stoplight/spectral-core/dist/runner/runner.js
+```
+
+It's a problem with Jest, which cannot understand [NodeJS's package exports](https://nodejs.org/api/packages.html). To fix that, should be [enabled ESM support in Jest](https://jestjs.io/docs/ecmascript-modules) or set an appropriate Jest's `moduleNameMapper` config:
+
+```js
+moduleNameMapper: {
+  '^nimma/legacy$': '<rootDir>/node_modules/nimma/dist/legacy/cjs/index.js',
+  '^nimma/(.*)': '<rootDir>/node_modules/nimma/dist/cjs/$1',
+},
+```
+
 ## Develop
 
 1. Write code and tests in the `test` folder.

test/models/v2/schema.spec.ts

@@ -92,15 +92,28 @@ describe('Channel model', function() {
       const d = new Schema(doc);
       expect(d.additionalItems()).toEqual(true);
     });
+
+    it('should return true when not defined', function() {
+      const doc: any = {};
+      const d = new Schema(doc);
+      expect(d.additionalItems()).toEqual(true);
+    });
+
+    it('should return false when null', function() {
+      const doc: any = { additionalItems: null };
+      const d = new Schema(doc);
+      expect(d.additionalItems()).toEqual(false);
+    });
   });
 
-  describe('.additionalProperties()', function() {
-    it('should return the value schema object', function() {
-      const doc = { additionalProperties: {} };
+  describe('additionalProperties()', function() {
+    it('should return a Schema object', function() {
+      const doc: any = { additionalProperties: { type: 'string' } };
       const d = new Schema(doc);
       expect(d.additionalProperties()).toBeInstanceOf(Schema);
+      expect((d.additionalProperties() as Schema).json()).toEqual(doc.additionalProperties);
     });
-
+    
     it('should return the true when there is no value', function() {
       const doc = {};
       const d = new Schema(doc);
@@ -112,12 +125,18 @@ describe('Channel model', function() {
       const d = new Schema(doc);
       expect(d.additionalProperties()).toEqual(false);
     });
-
-    it('should return the false where there is true value', function() {
-      const doc = { additionalProperties: true };
+    
+    it('should return true when not defined', function() {
+      const doc: any = {};
       const d = new Schema(doc);
       expect(d.additionalProperties()).toEqual(true);
     });
+
+    it('should return false when null', function() {
+      const doc: any = { additionalProperties: null };
+      const d = new Schema(doc);
+      expect(d.additionalProperties()).toEqual(false);
+    });
   });
 
   describe('.allOf()', function() {