Fix diffJson's handling of non-callable toJSON properties (#700)

* Add test showing behaviour reported in #699

* Fix #699

* Add release notes

* Release notes

Author: ExplodingCabbage

release-notes.md

@@ -3,6 +3,7 @@
 ## 9.1.0 (prerelease)
 
 - [#697](https://github.com/kpdecker/jsdiff/pull/697) *`diffJson` now correctly handles JSON objects containing a key named `__proto__`*. (Previously, the returned diff would be as if the `__proto__` key did not exist on either of the objects being diffed.)
+- [#700](https://github.com/kpdecker/jsdiff/pull/700) *`diffJson` now correctly handles JSON objects containing a non-callable property named `toJSON`* - i.e. it gives such a property no special behaviour whatsoever, just as `JSON.stringify` doesn't. Previously, such properties caused an error to be thrown. (*Callable* `toJSON` properties continue to get the same special behaviour that `JSON.stringify` gives them.)
 
 ## 9.0.0
 

src/diff/json.ts

@@ -97,7 +97,7 @@ export function canonicalize(
     return canonicalizedObj;
   }
 
-  if (obj && obj.toJSON) {
+  if (obj && typeof obj.toJSON === 'function') {
     obj = obj.toJSON();
   }
 

test/diff/json.js

@@ -127,6 +127,38 @@ describe('diff/json', function() {
         { count: 1, value: '}', removed: false, added: false }
       ]);
     });
+
+    it('handles custom toJSON methods like JSON.stringify does', function() {
+      const x = {
+        toJSON: () => 'aaa'
+      };
+      const y = {
+        toJSON: () => 'bbb'
+      };
+
+      expect(diffJson({ foo: x }, {foo: y})).to.eql([
+        { count: 1, value: '{\n', removed: false, added: false },
+        { count: 1, value: '  "foo": "aaa"\n', added: false, removed: true },
+        { count: 1, value: '  "foo": "bbb"\n', added: true, removed: false },
+        { count: 1, value: '}', removed: false, added: false }
+      ]);
+    });
+
+    it('treats non-callable toJSON properties as normal properties (like JSON.stringify does)', function() {
+      const x = {
+        toJSON: 'aaa'
+      };
+      const y = {
+        toJSON: 'bbb'
+      };
+
+      expect(diffJson(x, y)).to.eql([
+        { count: 1, value: '{\n', removed: false, added: false },
+        { count: 1, value: '  "toJSON": "aaa"\n', added: false, removed: true },
+        { count: 1, value: '  "toJSON": "bbb"\n', added: true, removed: false },
+        { count: 1, value: '}', removed: false, added: false }
+      ]);
+    });
   });
 
   describe('#canonicalize', function() {