ReactiveDict: can set absent value to undefined

Setting a ReactiveDict value to `undefined` does not remove it, and
undefined values are migrated.  Therefore, it is odd that setting
'foo' to `undefined` on the empty dict does not produce the dict
`{foo: undefined}`, but setting 'foo' first to another value and *then*
`undefined` does.  So, we fix that.

This means that setting a value to `undefined` blocks `setDefault` on
that key, which makes sense given that setting the value to 123 and then
`undefined` blocks `setDefault` on that key.

packages/reactive-dict/package.js

@@ -16,5 +16,6 @@ Package.onTest(function (api) {
   api.use('tinytest');
   api.use('reactive-dict');
   api.use('tracker');
+  api.use('underscore');
   api.addFiles('reactive-dict-tests.js');
 });

packages/reactive-dict/reactive-dict-tests.js

@@ -1,11 +1,19 @@
+Tinytest.add('ReactiveDict - set to undefined', function (test) {
+  var dict = new ReactiveDict;
+  dict.set('foo', undefined);
+  test.equal(_.keys(dict.all()), ['foo']);
+  dict.setDefault('foo', 'bar');
+  test.equal(dict.get('foo'), undefined);
+});
+
 Tinytest.add('ReactiveDict - all() works', function (test) {
   var all = {}, dict = new ReactiveDict;
   Tracker.autorun(function() {
     all = dict.all();
   });
-  
+
   test.equal(all, {});
-  
+
   dict.set('foo', 'bar');
   Tracker.flush();
   test.equal(all, {foo: 'bar'});
@@ -15,7 +23,7 @@ Tinytest.add('ReactiveDict - all() works', function (test) {
 Tinytest.add('ReactiveDict - clear() works', function (test) {
   var dict = new ReactiveDict;
   dict.set('foo', 'bar');
-  
+
   var val, equals, equalsUndefined, all;
   Tracker.autorun(function() {
     val = dict.get('foo');
@@ -29,16 +37,16 @@ Tinytest.add('ReactiveDict - clear() works', function (test) {
   Tracker.autorun(function() {
     all = dict.all();
   });
-  
+
   test.equal(val, 'bar');
   test.equal(equals, true);
   test.equal(equalsUndefined, false);
   test.equal(all, {foo: 'bar'});
-  
+
   dict.clear();
   Tracker.flush();
   test.isUndefined(val);
   test.equal(equals, false);
   test.equal(equalsUndefined, true);
   test.equal(all, {});
-});
+});

packages/reactive-dict/reactive-dict.js

@@ -50,6 +50,7 @@ _.extend(ReactiveDict.prototype, {
     var self = this;
 
     if ((typeof keyOrObject === 'object') && (value === undefined)) {
+      // Called as `dict.set({...})`
       self._setObject(keyOrObject);
       return;
     }
@@ -60,9 +61,11 @@ _.extend(ReactiveDict.prototype, {
     value = stringify(value);
 
     var oldSerializedValue = 'undefined';
-    if (_.has(self.keys, key)) oldSerializedValue = self.keys[key];
+    if (_.has(self.keys, key)) {
-    if (value === oldSerializedValue)
+      oldSerializedValue = self.keys[key];
-      return;
+      if (value === oldSerializedValue)
+        return;
+    }
     self.keys[key] = value;
 
     self.allDeps.changed();