add more test for mermaidAPI.parse() to ensure it matches documented behavior

src/mermaidAPI.spec.js

@@ -1,3 +1,4 @@
+import mermaid from './mermaid';
 import mermaidAPI from './mermaidAPI';
 import { assignWithDepth } from './utils';
 
@@ -119,12 +120,28 @@ describe('when using mermaidAPI and ', function () {
       expect(mermaidAPI.getConfig().dompurifyConfig.ADD_ATTR).toEqual(['onclick']);
     });
   });
-  describe('checking validity of input ', function () {
-    it('it should throw for an invalid definiton', function () {
+  describe('test mermaidApi.parse() for checking validity of input ', function () {
+    mermaid.parseError = undefined; // ensure it parseError undefined
+    it('it should throw for an invalid definiton (with no mermaid.parseError() defined)', function () {
+      expect(mermaid.parseError).toEqual(undefined);
       expect(() => mermaidAPI.parse('this is not a mermaid diagram definition')).toThrow();
     });
     it('it should not throw for a valid definiton', function () {
       expect(() => mermaidAPI.parse('graph TD;A--x|text including URL space|B;')).not.toThrow();
     });
+    it('it should return false for invalid definiton WITH a parseError() callback defined', function () {
+      var parseErrorWasCalled = false;
+      // also test setParseErrorHandler() call working to set mermaid.parseError
+      mermaid.setParseErrorHandler(function (error, hash) {
+        // got here.
+        parseErrorWasCalled = true;
+      });
+      expect(mermaid.parseError).not.toEqual(undefined);
+      expect(mermaidAPI.parse('this is not a mermaid diagram definition')).toEqual(false);
+      expect(parseErrorWasCalled).toEqual(true);
+    });
+    it('it should return true for valid definiton', function () {
+      expect(mermaidAPI.parse('graph TD;A--x|text including URL space|B;')).toEqual(true);
+    });
   });
 });