"attr" binding now removes attributes if the bound value is strictly …

…false (thanks to Greenlaw for providing this use case)

build/output/knockout-latest.debug.js

@@ -1467,7 +1467,10 @@ ko.bindingHandlers['attr'] = {
         for (var attrName in value) {
             if (typeof attrName == "string") {
                 var attrValue = ko.utils.unwrapObservable(value[attrName]);
-                if (attrValue === undefined) 
+
+                // To cover cases like "attr: { checked:someProp }", we want to remove the attribute entirely 
+                // when someProp===false (because that's how to mark an element as not checked, not disabled, etc.)
+                if (attrValue === false) 
                     element.removeAttribute(attrName);
                 else 
                     element.setAttribute(attrName, attrValue.toString());

spec/defaultBindingsBehaviors.js

@@ -694,7 +694,7 @@ describe('Binding: Attr', {
         value_of(testNode.childNodes[0].getAttribute("second-attribute")).should_be("true");
     },
 
-    'Should respond to changes in an observable value, removing the attribute if the value is undefined': function() {
+    'Should respond to changes in an observable value, removing the attribute if the value is strictly false': function() {
         var model = { myprop : ko.observable("initial value") };
         testNode.innerHTML = "<div data-bind='attr: { someAttrib: myprop }'></div>";
         ko.applyBindings(model, testNode);
@@ -705,7 +705,7 @@ describe('Binding: Attr', {
         value_of(testNode.childNodes[0].getAttribute("someAttrib")).should_be("new value");
 
         // Set to undefined; see the attribute vanish
-        model.myprop(undefined);
+        model.myprop(false);
         value_of(testNode.childNodes[0].getAttribute("someAttrib")).should_be(null);        
     }
 });

src/binding/defaultBindings.js

@@ -384,7 +384,10 @@ ko.bindingHandlers['attr'] = {
         for (var attrName in value) {
             if (typeof attrName == "string") {
                 var attrValue = ko.utils.unwrapObservable(value[attrName]);
-                if (attrValue === undefined) 
+
+                // To cover cases like "attr: { checked:someProp }", we want to remove the attribute entirely 
+                // when someProp===false (because that's how to mark an element as not checked, not disabled, etc.)
+                if (attrValue === false) 
                     element.removeAttribute(attrName);
                 else 
                     element.setAttribute(attrName, attrValue.toString());